Introduction

The interplay among spin, charge and orbital degrees of freedom in transition metal compounds has triggered enormous research interests in condensed matter physics and material science. For a large family of layered 3d electron superconductors (SCs) such as the copper oxides1 and iron pnictides2, the spin fluctuations caused by strong 3d electron correlations play a vital role in the unconventional superconductivity. Besides these 3d transition metal systems, several platinum-based SCs exhibit remarkably rich physical properties and therefore have also attracted considerable attention, partly owing to the moderately strong spin-orbit coupling of the platinum 5d electrons. The most prominent example is the heavy fermion noncentrosymmetric (NCS) SC CePt3Si, in which exotic superconductivity is observed below Tc = 0.75 K3: an admixture of spin-singlet and spin-triplet pairing symmetry, nodal gap structure and huge upper critical field (Bc2 ≈ 4 T)4. The delicate interplay between the cerium 4f and the platinum 5d electrons places this material on the border of the magnetic quantum critical point (QCP) but still in the antiferromagnetic (AFM) gound state, rendering the role of inversion symmetry unclear5. Among a series of filled skutterudite MT4X12 (M = rare-earth or alkaline-earth metals, T = transition metals and X = P, As, Sb and Ge) with the cubic space group (No. 204), PrPt4Ge12 was reported to exhibit time-reversal symmetry breaking from zero-field μSR measurements6. As a result of the unexpectedly high transition temperature Tc = 7.9 K and the moderately enhanced Sommerfeld coefficient γ = 76 mJ/mol ⋅ K2, PrPt4Ge12 has been extensively studied and multiband superconductivity has been proposed based on the analysis of the photoemission spectroscopy7 as well as the magnetic penetration depth8. Moreover, SrPtAs is recently reported to crystallize in a hexagonal structure (P63/mmc, No. 194) with weakly coupled PtAs layers forming a honeycomb lattice9. The peculiar locally NCS structure within PtAs layer together with a strong spin-orbit coupling demonstrates SrPtAs as an attractive material to explore superconductivity with a spontaneous static magnetic field Bs10.

It is interesting that among the platinum-based superconductors, the newly reported family of APt3P (A = Ca, Sr and La) shares the structural similarity with that of iron pnictides11. These compounds crystallize in a tetragonal structure with space group P4/nmm (No. 129) with stacking in the order of A-Pt6P-A along the c-axis. The distorted antiperovskite Pt6P octahedral unit alternates within the ab plane, forming an antipolar pattern. The z → −z inversion operation is thus preserved. Due to the structural distortion, the platinum atoms take two different sites as Pt(I) and Pt(II) so that the Pt(II) and P atoms form a Pt2P layer resembling the FeAs layer in the iron-based superconductors. Of course, the structure of APt3P is also somewhat similar to that of CePt3Si, but the latter is actually isotypic to the NCS compound CePt3B with the space group P4mm (No. 99)3. The corresponding Pt6Si unit has the polar structure under this space group leading to the absence of inversion symmetry, different from the antipolar structure in APt3P. Noticeably, the APt3P family shows a significant variation of Tc, i.e., Tc = 8.4 K, 6.6 K and 1.5 K for A = Sr, Ca and La, respectively. It was reported theoretically that spin-orbit coupling (SOC) effect is significant in LaPt3P but negligible in CaPt3P and SrPt3P12,13,14. The origin of significantly enhanced Tc in SrPt3P is still debatable. It was suggested to be due to a possible dynamic charge-density-wave (CDW)12. However, a theoretical work by Zocco et al. indicated SOC could strongly renormalize the electron-phonon coupling of SrPt3P and thus enhance the electronic density of states near the Fermi level15. Moreover, several theoretical works claimed that the CDW instability could not be reproduced in SrPt3P13,14. The centrosymmetric (CS) compounds APt3P reported so far do not involve the 4f electrons. The interplay between strong 4f electron correlation and superconductivity of 5d electrons in the APt3P family remains an open issue.

In this paper, we report our successful synthesis of such a candidate compound CePt3P in the platinum-based phosphides APt3P family. We performed systematic measurements of the physical properties including the magnetic susceptibility, magnetization, specific heat and electrical resistivity. However, no evidence of superconductivity is observed down to 0.5 K in CePt3P, in contrast to other APt3P compounds. Instead, the compound displays the rich physics involving the coexistence of magnetic ordering, Kondo coherence as well as crystalline electric field (CEF) effect. We shall discuss these properties and highlight the delicate 4f-5d interplay in this system.

Results and Discussion

Figure 1 shows the Rietveld refinement of the XRD pattern of polycrystalline CePt3P samples. Almost all peaks can be well indexed with the tetragonal structure with the space group P4/nmm (No. 129), except for a tiny peak of an impurity phase around 31.4° which might be PtP2. The result of the Rietveld refinement16 shows a good convergence: Rwp = 13.4%, S = 3.3. The refined lattice parameters of CePt3P are a = 5.7123(7) Å and c = 5.4679(6) Å as listed in Table 1. The room temperature XRD patterns of LaPt3P are also refined with Rwp = 14.9%, S = 2.7 (data not shown). The refined lattice parameters of LaPt3P are a = 5.7597(3) Å and c = 5.4736(3) Å. For comparison, the lattice parameters of the other APt3P compounds are also provided in Table 1. One can see obviously that a of CePt3P is smaller, while c is larger, compared with the lattice parameters of SrPt3P. Due to the lanthanide contraction, both of a and c of CePt3P are smaller than those of LaPt3P. From the EDS measurements, the molar ratio is Ce:Pt:P = (1.0 ± 0.1):(3.2 ± 0.2):(0.7 ± 0.2) for CePt3P and La:Pt:P = (1.0 ± 0.1):(2.6 ± 0.1): (0.8 ± 0.1) for LaPt3P. The actual chemical compositions are close to the nominal ones, while there seems a deficiency on the P site for both CePt3P and LaPt3P.

Figure 1: Rietveld refinement of the polycrystalline CePt3P XRD pattern.
figure 1

Arrow marks an impurity phase which might be PtP2. Inset shows a typical energy-dispersive x-ray spectrum with electron beams focused on the selected area of the as-grown sample.

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Table 1 Comparisons of physical parameters among the APt3P family with A = Sr, Ca, La and Ce.
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The temperature-dependent molar magnetic susceptibility χ(T) = M/H and inverse magnetic susceptibility 1/χ(T) of CePt3P measured at H = 1000 Oe are presented in Fig. 2(a). χ(T) obeys a modified Curie-Weiss law above 200 K, χ = χ0 + /(T − θ). χ0 is a temperature independent susceptibility from the core diamagnetism, the van Vleck and Pauli paramagnetism, is the Curie constant and θ is the Weiss temperature. The relatively large absolute value of θ = −28.3 K may be attributed to the hybridization of the 4f electronic states with the conduction band17. The derived effective moment μeff = 2.52μB is almost equal to that of a free Ce3+ ion, indicating the trivalent Ce ion and well localized moment of Ce-4f1 electrons at high temperature. χ0 is in the magnitude order of 10−3. For T < 100 K, a change of the slope of 1/χ(T) can be clearly observed and the fitting parameters are μeff = 2.11μB, and θ = −15.3 K. Here the change of the slope and the decreased value of μeff can be ascribed to the CEF effect. With decreasing temperature, χ(T) increases and shows a round peak around 3.0 K. Upon further cooling, another anomaly is observed near our base temperature. Two magnetic transition temperatures are determined from the peaks of derivative susceptibility Tdχ/dT as TN1 = 3.0 K and TN2 = 1.9 K (seen from Fig. 2(b)). Considering the negative Weiss temperature, the first anomaly marks the AFM ordering below TN1 which is compatible with the magnetization measurement (discussed below). While the second anomaly is attributed to a spin-reorientation. A similar phenomenon was observed in CeNiAsO18. Further experimental studies, especially neutron diffraction measurement on single crystals of CePt3P, are necessary to clarify the magnetic structure at low temperature.

Figure 2
figure 2

(a) Temperature dependence of magnetic susceptibility, χ, and inverse magnetic susceptibility, 1/χ, of CePt3P measured under magnetic field B = 0.1 T on the left and right axis, respectively. Two dashed lines show the Curie-Weiss fit for T > 200 K and T < 100 K, respectively. Inset: enlarged plot of χ at T < 5 K. (b) The AFM transition temperature TN1 and TN2 determined from the derivative susceptibility Tdχ/dT, specific heat C(T)/T and derivative resistivity dρ/dT (The complete data of specific heat and resistivity will be shown in the following figures).

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The isothermal magnetization M(B) of CePt3P, measured in the B-sweep mode containing both field-up and down loops, is displayed in Fig. 3(a). In the AFM ordering state, M(B) displays a linear field dependence when B < 2.0 T, but undergoes a weak step-like increase around 3.0 T. This anomaly, which is ascribed to a field-induced metamagnetic transition (MMT), can be independently determined to be Bm = 3.0 T by the peak in dM/dB curve (inset to Fig. 3(a)) and the hump in ρ(B) curve (Fig. 3(b)) measured at T = 2 K. The expected hysteresis around Bm is not observed and such absence of hysteresis around MMT was also reported in the single-crystalline samples CeAuSb219 and YbNiSi320. No hysteresis in resistivity is observed for CePt3P in this magnetic field range either. Note that the M(B) curve does not show a saturation trend in the highest field limit and the value M ~ 0.6μB at B = 5 T is much lower than the theoretical value of 2.14μB for the saturated moment of free Ce3+ ions which is probably due to the CEF effect. Figure 3(b) shows the isothermal resistivity versus the applied field. ρ decreases monotonously with increasing magnetic field at T = 6 K > TN1. Whereas at T = 2 K < TN1, a hump around Bm = 3.0 T is added to the decreasing trend. This feature is compatible with the MMT observed in the magnetization measurement.

Figure 3
figure 3

(a) Field dependence of the magnetization M(B). (b) Resistivity ρ of CePt3P vs. B measured at T = 2 and 6 K. Inset to (a) displays the derivative of the magnetization with respect to the field dM/dB for T = 2 K.

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The specific heats of CePt3P and LaPt3P divided by T, C(T)/T, are plotted in the main panel of Fig. 4(a) in a semi-logarithm scale. At room temperature, C(T) saturates to about 135 and 140 J/mol ⋅ K for La and Ce compound, respectively, which are, within an acceptable error range, compatible with the classical Dulong-Petit law 3NR with N = 5 and R = 8.31 J/mol ⋅ K, where R is the universal gas constant. The specific heat C(T) of LaPt3P is typical for nonmagnetic metals since no typical anomaly can be observed at high temperature. At low temperature, the specific heat of LaPt3P is dominated by the electronic and phonon contributions for T < ΘD/10, therefore, it can be fitted to a power law C/T = γLa + βLaT2 over 10–20 K (data not shown). Here ΘD is the Debye temperature, and γLa and βLa denote the coefficients of the electronic and phonon contributions, respectively. It should be noted that there is a small jump around 1 K in the specific heat of LaPt3P which should correspond to a superconducting transition though it is too small to observe in Fig. 4.

Figure 4
figure 4

(a) Specific heat divided by temperature, C/T, versus logT. The solid symbols are for CePt3P, while the open symbols represent the non-magnetic compound LaPt3P. The solid line is a fit to Eq. 5 for T ≤ 1.9 K. (b) The Ce-4f contribution, C4f/T, and the magnetic entropy, Sm, on the left and right axis, respectively, measured at zero magnetic field plotted in a logarithmic temperature scale for T = 0.4–200 K. The solid line shows the Schottky anomaly contribution CSch. Inset to (a) shows C/T versus T2 together with the fitting parameters for CePt3P (see the text): the Sommerfeld coefficient γCe, βCe and the Debye temperature ΘD. The dashed line is a linear fit in the temperature range T = 10–20 K. Inset to (b) displays the schematic sketch of CEF energy levels for Ce3+ ion in CePt3P.

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In the paramagnetic region above the magnetic transition, the specific heat of CePt3P can be expressed as

where the coefficients γCe and βCe are of electronic and phonon contributions of CePt3P, respectively, while CSch describes the Schottky anomaly item. A linear T2-dependence is clearly seen in C/T vs T2 plot for temperature below 20 K (see inset to Fig. 4(a)). The derived Sommerfeld coefficient is γCe = (86 ± 1) mJ/mol ⋅ K2. The value is moderately enhanced by a factor of 57 compared with that of LaPt3P where γLa = (1.5 ± 0.1) mJ/mol ⋅ K2, manifesting the correlation effect contributed from the Ce-4f electrons. Therefore, CePt3P is a Kondo lattice compound due to the strong 4f electron correlation and moderate effective 4f−5d hybridization. Note that γLa for LaPt3P derived here is slightly smaller but still in the same magnitude order with that obtained in ref. 11. The reported phonon coefficients are in reasonable agreement with each other: βCe = 0.98(1) mJ/mol ⋅ K4 for CePt3P and βLa = 0.94(1) mJ/mol ⋅ K4 for LaPt3P, indicating similar phonon contributions. The Debye temperature ΘD estimated by using ΘD = (12π4NR/5β)1/3 is (215 ± 1) K for CePt3P and (218 ± 1) K for LaPt3P, implying that the above analysis is quite self-consistent.

The Ce-4f contribution to the specific heat of CePt3P is then deduced by subtracting the measured specific heat of the nonmagnetic isostructural reference sample LaPt3P from the total specific heat of CePt3P, i.e., C4f = CCe − CLa. The result is shown in the main panel of Fig. 4(b), plotted as C4f/T vs T in a logarithmic scale. The Schottky anomaly, which is visible as a broad peak centered around 90 K in C4f/T curve, should be caused by the excitations between different CEF levels. The Schottky anomaly with three Kramers doublets (one doublet ground state and two excited doublets) for Ce3+ ion with j = 5/2 experiencing a tetragonal crystal-field potential can be expressed by refs 21,22

Here gi = 2 is the degeneracy of the ith doublet state and Δi is the energy difference between the ground state and the i-th excited state (see the schematic sketch drawn in the inset of Fig. 4(b)). Eq. 2 is applied to C4f/T of CePt3P over a temperature range of 50–130 K. The derived CEF energy differences are Δ1 = (20.9 ± 0.1) meV (~(240 ± 1) K) and Δ2 = (60.9 ± 0.3) meV (~(700 ± 3) K). This result may explain the slope change in 1/χ(T) curve as well as the broad hump in both ρmag and S. Furthermore, the large value of Δ1 is consistent with the reduced effective Ce moment below 100 K. The magnetic entropy gain Sm is calculated by integrating C4f/T over T and plotted on the right axis in Fig. 4(b). One can see that Sm reaches about 0.51Rln2 at TN1 and Rln2 is recovered at ~50 K, indicating that the ground state with the AFM ordering of Ce3+ moments is Kramers two-fold degenerate. The plateau over the temperature range of T = 10–30 K indicates that the first excited CEF level is far above TN1. Sm reaches Rln4 at ~150 K and increases substantially above the Schottky anomaly. For a Kondo lattice, the Kondo temperature can be estimated by the magnetic entropy at TN via ref. 23

where ξ = TK/TN. The yielded TK is about (6.1 ± 0.1) K for CePt3P.

At low temperature, C4f/T shows a pronounced λ-shape peak at TN1 = 3.0 K, implying a second-order phase transition. The expected jump in specific heat is ~6 J/mol ⋅ K. A slight slope change in C4f/T is also observed around TN2 = 1.9 K, consistent with the low-temperature anomaly observed in aforepresented χ(T) curve. Based on the mean-field theory of Besnus et al.24 and Bredl et al.25, the specific heat jump is related to the Kondo temperature TK by the following formula

Here ζ = (TK/TN)/2π, ψ denotes the digamma function and ψ′, ψ′′ and ψ′′′ are the first three derivatives of ψ. Then the Kondo temperature can be also estimated by applying Eq. 4, obtaining a ratio of TK/TN1 = 0.88, or TK ~ (2.7 ± 0.1) K. Therefore, based on both magnetic entropy and specific heat jump, it is reasonable to estimate TK ~ 2–6 K in this compound.

In the magnetically ordered state, the AFM spin-wave spectrum follows a dispersion relation of εk =. Here εk is the excitation energy, Δ is the gap in the spin-wave spectrum, and D is the spin-wave stiffness. The phonon contribution, βCeT3 item, can be subtracted from the total specific heat C as ΔC = C − βCeT3. At low temperature, ΔC is described by the following expression26,27:

where the coefficient AC is proportional to D−3. Fitting the specific heat below TN2 (solid line in Fig. 4(a)) gives the fitting parameters γ0 = 247 mJ/mol ⋅ K2, Δ = 2.6 K, and AC = 67.5 mJ/mol ⋅ K4. The considerably enhanced zero-temperature Sommerfeld coefficient γ0 is about 3 times of γCe obtained in the paramagnetic state, indicating the formation of moderate-heavy quasiparticles in the antiferromagnetically ordered state. It is worthwile noting that the obtained spin-wave gap Δ is of the order of magnitude often found in cerium intermetallics with AFM ground states17.

The temperature variation of the electrical resistivity of CePt3P, ρ(T), is plotted in Fig. 5(a). The resistivity at room temperature is ρ300K = 1140 μΩ ⋅ cm, a value rather typical for the Ce-based Kondo compounds with narrow f-band28. The resistivity decreases with decreasing temperature and exhibits two features. A broad hump around 110 K reflects the 4f-electron contribution via Kondo scattering from different CEF levels21,22. At low temperature, a pronounced peak in ρ(T) around 3 K is directly visible, indicating the AFM ordering phase below TN1 = 3.0 K. Above TN1, ρ increases in a minus logarithmic temperature manner over T = 5–20 K, reflecting the Kondo-type scattering. Further evaluation of ρ(T) requires information of the phonon contribution which could be taken from the homologous and isostructural analog, LaPt3P. The ρ(T) of LaPt3P, which is also presented in Fig. 5(a), can be well described by a Bloch-Grüeneisen-Mott (BGM) relation:

Figure 5: Transport properties as a function of temperature.
figure 5

(a) ρ(T) of CePt3P and LaPt3P in a linear temperature scale. The solid line is a fit to the Bloch-Grüneissen-Mott formula (Eq. 6). (b) The magnetic contribution to the electrical resistivity of CePt3P, ρmag, versus logT. The dashed lines display linear fits in the low and the high temperature regions, respectively. Inset to (a) plots a fit to Eq. 8 below T ≤ 1.9 K.

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where ρ0 is the residual resistivity due to lattice defects, the second term denotes electron-phonon scattering, and the third one accounts for the contribution due to Mott’s s-d interband electron scattering. A least square fitting of the BGM formula to the experimental data over the temperature range 2–300 K leads to the following parameters: ρ0 = 32 μΩ ⋅ m, ΘR = 160 K, R = 1.25 μΩ ⋅ cm/K, and K = 4.1 × 10−8 μΩ ⋅ cm/K3. Note that the residual resistivity ρ0 is smaller than that in ref. 11. The parameter ΘR is usually considered as an approximation of the Debye temperature ΘD in spite of some contribution due to electron-electron correlations in ΘR29. ΘD yielded from the specific heat data is 218 K which is in accordance with ΘR from the resistivity data. LaPt3P exhibits simple metallic behavior as we expected, without the characteristic features due to the interplay of Kondo and CEF effects in CePt3P mentioned above.

In order to analyze the magnetic contribution to the electrical resistivity of CePt3P, it is reasonable to assume that the phonon contribution in this compound can be properly approximated by that in LaPt3P, ρph = ρ(La) − ρ0(La), so we have

The temperature dependence of ρmag + ρ0 derived in this way is presented in Fig. 5(b) in a semilogarithmic scale. As a distinct feature in a Kondo lattice system, a pronounced broad hump centered at T* = 110 K become obvious in ρmag curve, which could be ascribed to the Kondo scattering from different CEF levels. According to Cornut and Coqblin21, this maximum provides an estimate of the CEF splitting energy scale ~200 K of Ce-4f1 state with j = 5/2. On the other hand, as temperature is decreased, ρmag increases in a logarithmic scale, as shown as the dotted lines in Fig. 5(b) above T > 200 K and between 5–20 K, respectively. Following the theoretical predictions of Cornut and Coqblin21, the logarithmic slopes and in the low-temperature and high-temperature regions, respectively, are proportional to the squared effective degeneracy λ of the thermally populated levels: cK ∝ λ2 − 1. For cerium compouns with Ce3+ ion placed in a noncubic crystalline environment the ground multiplet splits into three doublets, thus the expected ratio is :  = 3:35. In the case of CePt3P, with the coefficients  = −0.063 and  = −0.57 yielded from linear fitting of ρmag vs logT (see the dashed lines in Fig. 5(b)), the ratio is about 3:27, reasonably close to the theoretical prediction.

From the inset of Fig. 5(a), ρ drops rapidly below about 3.0 K owing to the reduction of spin-flip scattering upon entering the AFM ordered state. This magnetic transition temperature is determined from a slope change of dρ/dT in Fig. 2(b). Upon further cooling, a second slope change in ρ is observed around 1.9 K, corresponding to the pronounced kink in dρ/dT. Therefore, two magnetic transitions in CePt3P are apparent from the analysis of magnetic susceptibility χ(T), specific heat C(T) and electrical resistivity ρ(T), as shown in Fig. 2(b): the first transition TN1 corresponds to the AFM ordering temperature, while the second one TN2 is presumably associated with the spin reorientation. The values of TN1 and TN2 derived from different measurements agree well with each other. It is noted that while LaPt3P shows superconductivity around Tc = 1.0 K (from specific heat), no superconductivity is observed in CePt3P down to 0.5 K.

Considering the relativistic dispersion relation for the AFM magnon spectrum, the electrical resistivity ρ(T) for T < Δ can be well described by the following equations26,27:

where ρ0 is the temperature-independent residual resistivity, the constant coefficient Bρ is related to the spin-wave stiffness D by the proportionality D−3/2 and Δ is the same gap in the spin-wave spectrum as in Eq. (5). AT2 stems from the electron-electron scattering following the Fermi liquid theory, while the third term describes the electron-magnon scattering. This formula is applied to the electrical resistivity of CePt3P (dotted line in the inset of Fig. 5(a)) and a very good fit is obtained with the fitting parameters: ρ0 = 688 μΩ ⋅ cm, Δ = 4.0 K, A = 9.0 μΩ ⋅ cm/K2 and Bρ = 25 μΩ ⋅ cm/K2. Considering the relatively short fitting range of temperature, the derived Δ value for the measured polycrystalline sample is still reasonably compared with that obtained from the specific heat data.

Based on the above analyses, CePt3P displays the coexistence of three important characteristics: AFM ordering of the cerium local moments due to the Ruderman-Kittel-Kasuya-Yosida exchange interaction, the Kondo effect due to the strong 4f electron correlation and moderate effective 4f−5d hybridization, and the CEF interactions. The AFM ordering at TN1 = 3.0 K is clearly identified by the pronounced anomalies in the temperature-dependent magnetic, thermodynamic and electrical measurements. In addition, another anomaly at TN2 = 1.9 K is also visible from the physical properties, and is probably due to a change in the magnetic configuration within the AFM ordered phase. The behavior of ρ(T) and C(T) in the ordered region is well describable in terms of AFM spin-wave spectrum. The field-dependent behavior of the magnetization and electrical resistivity also indicates a MMT from the magnetic ordering to a spin-polarized state around Bm = 3.0 T. The magnetic structure of CePt3P is still unclear and the neutron diffraction or Mössbauer spectroscopy experiments are helpful to clarify the details of the magnetic structure.

The Kondo effect displays itself by the large value of Weiss temperature θ (compared with the ordering temperature), the reduced magnetic entropy and the specific heat jump at TN, as well as the enhanced Sommerfeld coefficient γCe. From the analysis of the specific heat data, the Kondo temperature TK is estimated to be in the range of 2–6 K. Its value can be also estimated from the magnetic susceptibility as TK ~ |θ|/4  7.1 K30, in reasonable agreement with other estimates. Also, the Kondo effect is well manifested in the electrical resistivity for Kondo systems with strong CEF interactions which follows the negative logarithmic-temperature dependence as ρ(T) = ρ0 + , with Kondo coefficient ck < 021. The inverse susceptibility (1/χ(T)) curve shows a slope change between T = 100–200 K which is also attributed to the CEF effect. This temperature region is in accordance with the energy scale Δ1 = 240 K of the multiplet Ce3+ ion estimated from the Schottky contributions of the specific heat21,22.

Finally, it is very interesting to compare this CS compound CePt3P with the extensively studied NCS heavy fermion SC CePt3Si (Tc = 0.75 K)3. The crystal structure of CePt3P consists of alternative stacking of layers of Ce atoms and layers of distorted antiperovskite Pt6P octahedral units along the c-axis. The Pt6P octahedra is asymmetrically distorted perpendicular to the ab-plane but alternatively distributed in the ab-plane, resulting in a symmetric antipolar analogue of CePt3Si. CePt3Si shows antisymmetric spin-orbit coupling of the platinum 5d electrons due to the absence of z → −z symmetry as well as mixing spin-singlet and spin-triplet pairing states. The parity mixing alone can hardly account for the heavy fermion phenomena unless the strong electron-electron correlation effects which are ensured by the presence of Ce3+ ions are taken into consideration together31. Correspondingly, the suppression of superconductivity in CePt3P may be attributed to the enhanced AFM ordering. CePt3P is, therefore, probably placed further away from the magnetic QCP compared with CePt3Si (TN = 2.2 K). With an external control parameter δ, such as doping or positive pressure, the system may be shifted towards TN = 0, namely the QCP32,33. It is thus of great interest to investigate whether superconductivity exists in CePt3P at even lower temperature than 0.5 K; if superconductivity does exist, it will provide strong evidence for the proximity to a magnetic QCP in CePt3P. Comparing with CePt3P, the occurrence of superconductivity at Tc = 0.75 K in CePt3Si implies that the NCS crystal structure may favor unconventional superconductivity within the AFM ground state.

Conclusion

In summary, we report the successful synthesis of a new compound CePt3P. From the collected experimental data of magnetization, specific heat and transport measurements, this compound is characterized as an antiferromagnetic Kondo lattice with crystal electric field effect. Two successive magnetic transitions of Ce 4f moments are observed: the magnetic ordering at TN1 = 3.0 K and the spin reorientation at TN2 = 1.9 K. Considering the moderately enhanced Sommerfeld coefficient of γCe = 86 mJ/mol ⋅ K2 in the paramagnetic region and large value of γ0 = 247 mJ/mol ⋅ K2 in the the AFM region, the Kondo effect and the AFM order should coexist in the ground state. Thus a relatively large Fermi surface formed by the heavy quasiparticles is expected in CePt3P with a Kondo temperature TK ~ 2–6 K. The ab initio crystal-field and electronic band structure calculations are necessary to further complement the present results. Further experiments such as chemical doping are presently underway in order to tune the ground state from the AFM ordering to strongly-correlated paramagnetic region.

Experimental Methods

The polycrystalline sample of CePt3P was synthesized by solid state reaction. Ce piece (99.8%), Pt powder (99.9%) and P lump (99.999%) of high purity from Alfa Aesar were used as starting materials. Firstly, CeP was pre-synthesized by reacting Ce and P at 1173 K for 72 h. Secondly, powders of CeP and Pt were weighed according to the stoichiometric ratio, thoroughly ground and pressed into pellets. The pellets were then packed in Al2O3 crucibles and sealed in an evacuated quartz tube which were slowly heated to 1273 K and kept at that temperature for 7 days. Finally, the samples were thoroughly ground, cold pressed and annealed in vacuum to improve the sample homogeneity. For comparison, the polycrystalline sample LaPt3P was also synthesized in the similar process. All the preparation procedures except heating were carried out in an argon protected glove box with the water and oxygen content below 0.1 ppm. The obtained CePt3P sample is less compact than LaPt3P and both of them are quite stable in the air.

Powder x-ray diffraction (XRD) measurements at room temperature were carried out on a PANalytical x-ray diffractometer (Model EMPYREAN) with a monochromatic Cu Kα1 radiation and a graphite monochromator. Lattice parameters were derived by Rietveld refinement using the program RIETAN 200016. The energy dispersion x-ray spectroscopy (EDS) analysis was performed on a EDS spectrometer affiliated to a field emission scanning electron microscope (FEI Model SIRION). The electron beam was focused on a crystalline grain and the chemical compositions were averaged on at least 4 EDS spectra from different grains. The electrical resistivity ρ(T) was measured by the standard four-probe method in a Quantum Design physical property measurement system (PPMS-9). The dc magnetization was measured in a Quantum Design magnetic property measurement system (MPMS-5) with the temperature range of T = 2-400 K. The specific heat measurements were performed in the PPMS-9 down to about 0.5 K.

Additional Information

How to cite this article: Chen, J. et al. Antiferromagnetic Kondo lattice compound CePt3P. Sci. Rep. 7, 41853; doi: 10.1038/srep41853 (2017).

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