External stimulation-controllable heat-storage ceramics

Commonly available heat-storage materials cannot usually store the energy for a prolonged period. If a solid material could conserve the accumulated thermal energy, then its heat-storage application potential is considerably widened. Here we report a phase transition material that can conserve the latent heat energy in a wide temperature range, T<530 K and release the heat energy on the application of pressure. This material is stripe-type lambda-trititanium pentoxide, λ-Ti3O5, which exhibits a solid–solid phase transition to beta-trititanium pentoxide, β-Ti3O5. The pressure for conversion is extremely small, only 600 bar (60 MPa) at ambient temperature, and the accumulated heat energy is surprisingly large (230 kJ L−1). Conversely, the pressure-produced beta-trititanium pentoxide transforms to lambda-trititanium pentoxide by heat, light or electric current. That is, the present system exhibits pressure-and-heat, pressure-and-light and pressure-and-current reversible phase transitions. The material may be useful for heat storage, as well as in sensor and switching memory device applications.

P hase transition phenomena, such as metal-insulator, ferroelectric ferromagnetic, and spin transitions, are attractive issues in the fields of physics, chemistry and materials science. Phase transitions are controlled not only by temperature change but also by other external stimuli such as pressure, light-irradiation or electric current flow. For example, for pressure-induced phase transitions, pressure-induced metal-semiconductor transition in a molybdenum disulphide 1 , pressure-induced superconductor transition in a fulleride 2 and pressure-induced ferroelectric-antiferroelectric transition in a perovskite system 3 have been reported. For light-induced phase transitions, light-induced crystalline-amorphous transitions in chalcogenides 4,5 , light-induced metal-semiconductor transition in a trititanium pentoxide 6 and insulator-metal transition in perovskite manganites 7,8 , light-induced spin-crossover transitions in metal complexes [9][10][11][12] and light-induced charge-transfer transition in organic molecules 13,14 and metal complexes 15 have been reported. Furthermore, for current-induced phase transitions [16][17][18] , current-induced insulator-metal transition in organic compound and current-induced magnetic-domain-wall switching in gallium manganese arsenide have been reported.
In recent years, heat-storage materials have been attracting attention from the viewpoint of energy saving. Development of high-performance heat-storage materials is important for the effective use of waste heat from blast furnaces in factories. Phase transition materials are considered to be useful as latent heat-storage materials. These are divided into solid-liquid and solid-solid phase transition types. In the former, the phase transition at the melting point (m.p.) is used for the heat storage. For example, water (320 kJ L À 1 at m.p. ¼ 0°C), paraffin (140 kJ L À 1 at m.p. ¼ 64°C) 19 and polyethylene glycol (165 kJ L À 1 at m.p. ¼ 20°C) 20 are known. In these cases, there are concerns of liquid spill from the system and mixing (or reaction) with the surrounding media. From this angle, a solidsolid phase transition material is stiff and its form is maintained without support, while at the same time it has chemical stability against the surrounding media. Well-known solid-solid phase transition materials for heat-storage usage include copolymers (for example, hyperbranched polyurethane: 150 kJ L À 1 at 67°C) 21 , organic compounds (for example, neopentylglycol: 165 kJ L À 1 at 48°C and pentaerythritol: 360 kJ L À 1 at 188°C) 22,23 and organometallic compounds (for example, bis(n-hexadecylammonium) tetrachlorozincate: 120 kJ L À 1 at 103°C and bis(n-decylammonium) tetrachlorocuprate: 60 kJ L À 1 at 34°C) 19,24,25 . In general, such phase change heatstorage materials cannot store the energy for a prolonged period below the phase transition temperature. If a solid material could conserve the accumulated thermal energy and release it only on demand, then its heat-storage application potential is considerably widened. From this angle, our work focused on a phase transition where the latent heat of thermal phase transition could be stored.
In this paper, we report a heat-storage material composed of lambda-trititanium pentoxide. The solid-solid phase transition of this material can be controlled by heat, pressure application, light-irradiation and current flow. This heat-storage material can conserve a high accumulation of energy and release it by the application of a remarkably small external pressure.

Results
Material and morphology. The sample of the titanium oxide, a new series of lambda-trititanium pentoxide (l-Ti 3 O 5 ), was produced by sintering rutile-TiO 2 particles in a hydrogen atmosphere (see Methods). Elemental analysis using inductively coupled plasma mass spectrometry confirms that the formula of the material is Ti 3 O 5 . Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the obtained sample show a coral-like morphology with particle size of B4 Â 1 mm ( Supplementary Fig. 1), composed of aggregates of rectangular-shaped nanorods, of which the majority are B200 Â 30 nm dimensions (hereafter called 'stripe-type-l- Fig. 1a). The high-resolution TEM (HRTEM) image is shown in Fig. 1b. The Fourier transform analysis of the HRTEM image showed that the growth direction of the nanorods is along the crystallographic b axis. The atomic level image from HRTEM corresponds to the visualized electron density distribution map on the bc plane calculated by the maximum entropy method (MEM; Fig. 1c), described later.
Pressure-induced phase transition. X-ray powder diffraction (XRPD) measurements were performed to investigate the pressure (P) dependence of the crystal structure of the stripetype-l-Ti 3 O 5 . The XRPD pattern at 300 K under atmospheric pressure (P ¼ 0.1 MPa) is shown in Fig. 1d and Supplementary  Fig. 2. Rietveld analysis indicates that this sample is composed of 80.0(2)% l-Ti 3 O 5 and 20.0(2)% b-Ti 3 O 5 . l-Ti 3 O 5 adopts a monoclinic crystal structure (space group C2/m) with lattice parameters of a ¼ 9.83119 (19) (7), and a unit cell volume, V ¼ 371.207(12) Å 3 . l-Ti 3 O 5 has three symmetry-inequivalent Ti sites, Ti(1), Ti(2) and Ti(3), and five-symmetry-inequivalent O sites, O(1) to O(5). All the Ti sites form a six-coordinate structure. In the previous investigation 6 of the same polymorph prepared from anatase-TiO 2 nanoparticles, we observed some indications of a pressure effect. In the present research, the sample was pressed at various external pressures with a pellet press, and XRPD patterns were measured for the pellets after pressure release. With increasing P, the intensity of the XRPD peaks of l-Ti 3 O 5 decreased and those of b-Ti 3 O 5 increased ( Fig. 1d and Supplementary Fig. 3). The pressure where the fraction of l-Ti 3 O 5 becomes 50% (P 1/2 ) is B60 MPa as shown in Fig   ARTICLE energy in the system, heat capacity measurements were performed. First, we investigated the heat capacity of the pressure-produced b-Ti 3 O 5 . In the temperature region from 5 to 300 K, specific heat was measured by the relaxation technique using the physical properties measurement system (Fig. 3a), and above 300 K, specific heat accompanying the thermal phase transition from pressure-produced b-Ti 3 O 5 to l-Ti 3 O 5 was measured by differential scanning calorimetry (DSC; Fig. 3b). By combining the results from the physical properties measurement system and DSC measurements and integrating with temperature, the experimental enthalpy (H) curves of l-Ti 3 O 5 and b-Ti 3 O 5 versus temperature were obtained up to 600 K ( Fig. 3c; see Methods). The transition enthalpy (DH) associated with the firstorder phase transition from b-Ti 3 O 5 to l-Ti 3 O 5 was 230 ± 20 kJ L À 1 (12±1 kJ mol À 1 ). In the temperature decreasing process of the DSC measurement, there was no peak, indicating that the accumulated heat energy of the phase transition from b-Ti 3 O 5 to l-Ti 3 O 5 was conserved in the system.
Next the released energy of the pressure-induced phase transition from stripe-type-l-Ti 3 O 5 to b-Ti 3 O 5 was measured using a high-pressure micro-DSC measurement system at room temperature. After applying pressure, heat energy of 240 ± 40 kJ L À 1 was released, which almost corresponds to the heat accumulated energy (Fig. 3d). Therefore, this material conserves the heat energy of the phase transition from pressure- Thermal conductivity and sensible heat-storage performance. Bricks and concrete are useful as sensible heat-storage materials 20,26-28 since they release thermal energy slowly. Thermal conductivity measurements were performed for the stripe-type-l-Ti 3 O 5 and pressure-produced b-Ti 3 O 5 . The thermal conductivities were 0.20±0.02 W m À 1 K À 1 and 0.41±0.02 W m À 1 K À 1 for l-Ti 3 O 5 and b-Ti 3 O 5 , respectively, which are similar to the values of bricks (for example, 0.16 W m À 1 K À 1 ) 26 and concrete (for example, 0.57 W m À 1 K À 1 ) 28 .
Current-induced and light-induced phase transitions. Electric current was flowed to the pressure-produced b-Ti 3 O 5 sample at 298 K. By flowing a current of 0.4 A mm À 2 , the colour of the sample changed from brown to dark blue (Fig. 4a). The XRPD patterns before and after flowing the current indicate that b-Ti 3 O 5 is transformed into l-Ti 3 O 5 (Fig. 4b and Supplementary Movie 4). The electric current dependence on the conversion from the pressure-produced b-Ti 3 O 5 to l-Ti 3 O 5 shows that the threshold current value of the current-induced phase transition is 0.2 A mm À 2 (Supplementary Fig. 9). The origin of this current-induced phase transition is regarded as breaking of charge ordering or (and) Joule heat [16][17][18] . The mechanism by breaking of charge ordering is considered as follows: b-Ti 3 O 5 is a charge-localized state whose charge is localized on Ti 3 þ (3) with empty orbital on Ti 4 þ (2). In contrast, l-Ti 3 O 5 is a chargedelocalized state whose charge is delocalized on Ti (2) (Fig. 5a (i)). On the contrary, on applying external pressure, the G versus x curves change; for example, the energy barrier disappears o400 K when P is 60 MPa, and hence, l-Ti 3 O 5 transforms into b-Ti 3 O 5 on applying pressure (Fig. 5a (ii)). The x versus temperature curves of P ¼ 0.1 MPa and P ¼ 60 MPa are shown in Fig. 5b. As shown in Fig. 5c, x versus pressure plots indicate the threshold of the pressure-induced phase transition. The origin of the pressureinduced phase transition is the PDV term of DH( ¼ DU þ PDV), where DU and DV are the changes of internal energy and volume, respectively. At such a low pressure, the pressure-induced change on DU is very small and negligible. In fact, the phonon mode calculation under external pressure shows that the pressureinduced change of DU is B1 Â 10 À 3 kJ mol À 1 at 60 MPa, which is two orders smaller compared with PDV ¼ 0.19 kJ mol À 1 . The pressure-induced change on DS is also very small and cannot contribute to the pressure-induced phase transition in the present system (see Methods, Supplementary Fig. 11 and Supplementary  Tables 1, 2). It is noted that the observed x versus P plots of Fig. 1e is somewhat gradual. This is explained by the presence of a distribution in the transition pressure of the Slichter and Drickamer model, which may be due to the crystal size distribution. We have simulated this gradual pressure-induced phase transition with a distribution of transition pressures ( Supplementary Fig. 12).
In summary, we report the first metal oxide capable of conserving the accumulated heat energy of a phase transition. Stripe-type-l-Ti 3 O 5 can store a large heat energy of 230 kJ L À 1 , and this energy can be released by applying external pressure only when demanded. The magnitude of the required pressure is extremely small, B60 MPa. This value is remarkably smaller than the typical pressures observed in the pressure-induced phase  [30][31][32][33][34][35] and metallic compounds [36][37][38][39][40][41] , for example, the pressure-induced phase transition from rutile-TiO 2 to baddeleyite-type TiO 2 at 1,043 K occurs at 20,000 MPa ( ¼ 20 GPa) 30 . From the viewpoint of the energy balance of the thermodynamic cycle, pressure of 60 MPa corresponds to B10 kJ L À 1 , which is o5% of the pressurereleasing heat energy. Pressure of B60 MPa can be realized even by the water pressure of a high-pressure washing machine, and hence, l-Ti 3 O 5 has the potential to be employed as pressuresensitive sheets or reusable portable heating pads. In addition, since l-Ti 3 O 5 is a metallic conductor and b-Ti 3 O 5 is a semiconductor, it has possibilities as a pressure-sensitive conductivity sensor or pressure-sensitive optical sensor. Furthermore, because l-Ti 3 O 5 is composed of common elements (titanium and oxygen), it is safe and environmentally friendly. l-Ti 3 O 5 could be useful for heat-retaining systems for residential use and may realize more efficient uses of industrial waste heat generated from furnaces ( Supplementary Fig. 13) 42,43 . In addition, light-induced and current-induced phase transitions from pressure-produced b-Ti 3 O 5 to l-Ti 3 O 5 are also observed, that is, stripe-type-l-Ti 3 O 5 shows reversible pressure-and-light-induced phase transition and reversible pressure-and-current-induced phase transition. These effects are also attractive phenomena from the viewpoint of advanced electronic devices.

Material.
A new series of l-Ti 3 O 5 nanocrystallites was produced by sintering rutile-TiO 2 particles in a hydrogen atmosphere (flow rates of 0.7 dm 3 min À 1 ) at 1,117°C for 2 h, followed by a slow cooling process of B9 h from the sintering temperature to room temperature ( Supplementary Fig. 14). Elemental analysis using inductively coupled plasma mass spectrometry confirms that the formula is Ti 3.00(1) O 5.00 (6) ; Calc.: Ti, 64.2%. Found: Ti, 64.2(1)%. The experimentally obtained density is 4.000 ± 0.048 g cm À 3 , which is consistent with the theoretical value of 4.00 g cm À 3 from the crystal structure of l-Ti 3 O 5 as determined by XRPD measurements. SEM and TEM images of the obtained sample show a coral-like morphology with particle size of B4 Â 1 mm, composed of rectangular-shaped nanorods, of which the majority are B200 Â 30 nm dimensions ( Supplementary  Fig. 1a). The Fourier transform analysis of the HRTEM image showed that the growth direction of the nanorods is along the crystallographic b axis. This new series of l-Ti 3 O 5 have larger crystal size than the previous series, which were prepared from anatase-TiO 2 (ref. 6; Supplementary Fig. 1b).
XRPD measurements. XRPD measurements were performed with a Rigaku Ultima IV diffractometer with Cu K a radiation (l ¼ 1.5418 Å). The temperaturedependent XRPD measurements were undertaken using a high-temperature chamber with atmosphere control (RIGAKU-OAT003S) under N 2 flow. The RIETAN-FP computer programme was used for the Rietveld analyses, while Dysnomia was used for the MEM analyses. The refined crystal structures and charge densities were visualized by the computer programme VESTA. Although both l-Ti 3 O 5 6 and its high-temperature phase 44,45 can be considered as candidates of the present material with C2/m crystal structure, we assigned the present material to l-Ti 3 O 5 because it is obtained by a very slow cooling process taking of ca. 9 h from the sintering temperature to room temperature, and it is thermally stable.
Heat capacity measurements. To investigate the temperature dependence of the lattice specific heat, C(T), in the temperature range of 5-300 K, we carried out curve fitting of the observed plots with the equation based on the two-Debye model 46 expressed by CðTÞ ¼ x 4 e x e x À 1 ð Þ 2 dx, where R is gas constant, c i is coefficient, y i is Debye temperature, x is ' o=k B T, : is the reduced Planck constant, o is phonon frequency and k B is Boltzmann constant, with the fit parameters of c 1 ¼ 3.2(1), c 2 ¼ 5.6(1), y 1 ¼ 4.1(1) Â 10 2 K and y 2 ¼ 9.3(1) Â 10 2 K for l-Ti 3 O 5 , and c 1 ¼ 2.7(1), c 2 ¼ 5.8(1), y 1 ¼ 4.3(1) Â 10 2 K and y 2 ¼ 9.3(2) Â 10 2 K for b-Ti 3 O 5 . We then developed the temperature dependence curve of the specific heat in the temperature range of 5-600 K using both the fitted curve and the anomalous specific heat associated with the first-order phase transition from b-Ti 3 O 5 to l-Ti 3 O 5 obtained from the DSC measurement.
Released heat energy on pressure application. Released heat energy on pressure application was measured with a high-pressure DSC measurement system (mDSC VII, SETARAM Instrumentation) at 300 K. Pressure application of 40 MPa was achieved by instant injection of N 2 gas into the sample cell. cutoff of 500 eV and 3 Â 7 Â 3 k-mesh until satisfying 10 À 5 eV pm À 1 force tolerance. Supercells (1 Â 3 Â 1) of the optimized structures were used to calculate the phonon modes and thermodynamic functions of l-Ti 3 O 5 and b-Ti 3 O 5 , which were calculated by the direct method implemented in Phonon code with 2 pm displacements using the optimized structures.
Thermodynamic analysis. In the Slichter and Drickamer mean-field model, the Gibbs free energy of the system is described as G ¼ x(DH) þ gx(1 À x) þ T{R[x lnx þ (1 À x)ln(1 À x)] À x(DS)} þ G b , where x is the ratio of the charge-delocalized unit of Ti(1) 3.3 þ À Ti(2) 3.3 þ À Ti(3) 3.3 þ corresponding to l-Ti 3 O 5 , g is the interaction parameter between l-Ti 3 O 5 and b-Ti 3 O 5 phases, G b is Gibbs free energy of b-Ti 3 O 5 set as the origin of the energies, and R is the gas constant. The observed phase transition was considered to be a metal-semiconductor phase transition between charge-delocalized Ti(1) 3.3 þ À Ti(2) 3.3 þ À Ti(3) 3.3 þ and charge-localized Ti(1) 3.0 þ À Ti(2) 3.7 þ À Ti(3) 3.3 þ systems, which were regarded as l-Ti 3 O 5 and b-Ti 3 O 5 , respectively. The values of DH ¼ 11.5 kJ mol À 1 and DS ¼ 25.2 J K À 1 mol À 1 , and a suitable value of g ¼ g a þ g b f(T ), where g a ¼ 14 kJ mol À 1 and g b ¼ 1.08 Â 10 À 2 J K À 1 mol À 1 to be consistent with the observation results, were used. When the external pressure is applied to the sample, DH is perturbed by the pressure-induced change on the DU and PDV terms. Compared with the pressure-induced change on the PDV term, for example, 0.19 kJ mol À 1 at P ¼ 60 MPa, the change on DU evaluated by the first-principles phonon mode calculations is negligibly small, for example, 1 Â 10 À 3 kJ mol À 1 at P ¼ 60 MPa. Thus, DH is controlled by the PDV term in the present system. The pressureinduced change on DS is also very small, for example, À 0.067 J K À 1 mol À 1 at P ¼ 60 MPa, from the results of first-principles phonon mode calculations.
Current-induced phase transition study. Stainless electrodes are attached to b-Ti 3 O 5 pellet by Ag paste with an adhesion area of 5 mm 2 and electric current of 2 A was flowed (0.4 A mm À 2 ) at 298 K. After that, the XRPD pattern of the surface of the pellet was measured.