Open AccessCCS ChemistryRESEARCH ARTICLE24 May 2022Pressure-Induced Intermetallic Charge Transfer and Semiconductor-Metal Transition in Two-Dimensional AgRuO3 Chuanhui Zhu, Jinjin Yang, Pengfei Shan, Mei-Huan Zhao, Shuang Zhao, Cuiying Pei, Bowen Zhang, Zheng Deng, Mark Croft, Yanpeng Qi, Lihong Yang, Yonggang Wang, Xiaojun Kuang, Long Jiang, Dao-Xin Yao, Jin-Guang Cheng and Man-Rong Li Chuanhui Zhu Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author , Jinjin Yang Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author , Pengfei Shan Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190 Google Scholar More articles by this author , Mei-Huan Zhao Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author , Shuang Zhao Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author , Cuiying Pei School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210 Google Scholar More articles by this author , Bowen Zhang College of Chemistry and Bioengineering, Guilin University of Technology, Guilin 541004 Google Scholar More articles by this author , Zheng Deng Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190 Google Scholar More articles by this author , Mark Croft Department of Physics and Astronomy, Rutgers, The State University of New Jersey, Piscataway, NJ 08854 Google Scholar More articles by this author , Yanpeng Qi School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210 ShanghaiTech Laboratory for Topological Physics, ShanghaiTech University, Shanghai 201210 Shanghai Key Laboratory of High-Resolution Electron Microscopy, ShanghaiTech University, Shanghai 201210 Google Scholar More articles by this author , Lihong Yang Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190 Google Scholar More articles by this author , Yonggang Wang Center for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing 100094 Google Scholar More articles by this author , Xiaojun Kuang College of Chemistry and Bioengineering, Guilin University of Technology, Guilin 541004 Google Scholar More articles by this author , Long Jiang Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author , Dao-Xin Yao Guangdong Key Laboratory of Magnetoelectric Physics and Devices, School of Physics, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author , Jin-Guang Cheng *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190 Google Scholar More articles by this author and Man-Rong Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202201989 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The intricate correlation between charge degrees of freedom and physical properties is a fascinating area of research in solid state chemistry and condensed matter physics. Herein, we report on the pressure-induced successive charge transfer and accompanied resistive evolution in honeycomb layered ruthenate AgRuO3. Structural revisiting and spectroscopic analyses affirm the ilmenite type R-3 structure with mixed valence cations as Ag+1/+2Ru+4/+5O3 at ambient pressure. In-situ pressure- and temperature-dependent resistance variation reveals a successive insulator-metal-insulator transition upon pressing, accompanied by unprecedented charge transfer between Ag and Ru under applied pressure, and a further structural phase transition in the insulator region at higher pressure. These phenomena are also corroborated by in-situ pressure-dependent Raman spectra, synchrotron X-ray diffraction, bond valence sums, and electronic structure calculations, emphasizing the dominated rare Ag2+, and near zero thermal expansion in the ab-plane in the metallic zone mostly due to the Jahn-Teller effect of d9-Ag2+. The multiple electronic instabilities in AgRuO3 may offer new possibilities toward novel and unconventionally physical and chemical behaviors in strongly correlated honeycomb lattices. Download figure Download PowerPoint Introduction Pressure-induced crystal and electronic structural transition of solids have proven a treasure trove for solid-state chemistry, condensed matter physics, and material science.1–3 In particular, for transition metal oxides with strong interplays between lattice, charge, spin, and orbital, pressure has demonstrated significant influence on the exotic phenomena in correlated systems, including magnetoelectric multiferroics, colossal magnetoresistance, and high temperature superconductivity.4–6 The perovskite BiNiO3 and ilmenite FeTiO3 are two typical compounds that exhibit pressure-induced intermetallic charge transfer.7,8 The electronic configurations between two metallic ions can be altered simultaneously via intermetallic charge transfer, giving rise to drastic changes in structure and physical properties.9–11 For instance, RCu3Fe4O12 (R = rare earth or Bi) displays an intermetallic charge transfer accompanied with paramagnetism-to-antiferromagnetism transitions, negative thermal expansion (NTE), and above room-temperature metal-to-insulator transition (MIT) as well.12–14 Mixed valence arising from intermetallic charge transfer has also been observed in 4d/5d oxides, such as in pressure-induced hexagonal perovskite Ba3BiM2O9 (M = Ru, Ir) and delafossite Cu2IrO3, reflecting the spatial dispersion of d orbitals.15,16 As one of the most appealing quantum materials, low-dimensional ruthenates have received growing attention due to their unique physical properties and quantum critical phenomena as reported in quasi-two-dimensional (2D) Ruddlesden-Popper phase, PbSb2O6-type analog, delafossite derivative, and halogenide.17,18 Particularly, honeycomb-layered ruthenates, which resemble the so-called Kitaev model with strong geometric frustration, are extremely interesting.19 The PbSb2O6-type SrRu2O6 shows high antiferromagnetic (AFM) ordering up to 565 K originated from the strong exchange interactions and large magnon gap.20,21 In delafossite derivatives, dimerized Li2RuO3 is a valence bond liquid with spin-singlet ground state and Cu3LiRu2O6 demonstrates pressure-induced nonmetallic-metal to bad-metal transition.22–24 α-RuCl3 is a hotspot in 2D ruthenates, because it demonstrates very strong exchange anisotropy and is a promising candidate for Kitaev quantum spin liquid, which is usually expected in a frustrated 2D system with S = 1/2 ions, such as d5-Ru3+ and d9-Ni+, Cu2+, and Ag2+.25–29 Recently, AgRuO3, which adopts an exotic perovskite-related ilmenite structure (R-3c), was reported to show an AFM transition around 342 K with semiconducting behavior.30,31 The honeycomb-layered RuO6 sheets are effectively separated by the larger size Ag ions, which, when compared with other conventional ilmenites, increases the interlayer distance, and renders strengthened 2D features in AgRuO3. However, the structure and physical properties of AgRuO3 under high-pressure (HP) remain unknown and worthy of further exploration. In this work, we revisited the crystal structure of AgRuO3 over a wide temperature range, and plotted the temperature- and pressure-dependent resistance phase diagram to understand this emerging compound in more depth. By combining various diffraction and spectroscopic techniques, we show that the ambient pressure (AP) AgRuO3 belongs to the R-3 space group with the charge formula as Ag+1/+2Ru+4/+5O3, instead of the reported R-3c. Upon pressurization, a successive insulator-metal-insulator and structural transition is ignited in the honeycomb lattice. Comprehensive experimental and theoretical structure investigations reveal a simultaneous charge transfer between Ag and Ru under applied pressure, accompanied with insulator-metal transition (IMT) at the Ag2+-dominated area. Further structural transitions to P2/m at higher pressure are also predicted. Experimental Methods Synthesis Samples of AgRuO3 were prepared via hydrothermal reaction in a Teflon-lined stainless steel autoclave following a literature report.31 A mixture of Ag2O (99.9%, Aladdin) and excess KRuO4 (98%, Strem) at a molar ratio of 1∶2.1 was added into distilled water (8 mL) and sonicated for 20 min. Subsequently, the mixture was sealed in a 25 mL Teflon-lined steel autoclave and heated at 423 K (heating rate of 5 K/min) for 24–72 h, and then cooled (cooling rate between 10 K/day and natural cooling) to room temperature. Optimal conditions were found by heating 72 h before natural cooling. The obtained samples were washed with deionized water and then ethanol before being dried at 323 K for 30 min. Finally, black crystals were obtained. Chemical and crystal structure characterizations The sample was initially characterized by single-crystal X-ray diffraction (SCXRD, SuperNova, MoKα radiation, λ = 0.71073 Å) at 150 K and 293 K. Further in-situ variable-temperature SCXRD data were collected from 100 K to 293 K (Bruker D8 Adventure Photon III (Guangzhou, Guangdong, China), GaInKα radiation, λ = 1.34138 Å). Powder XRD (PXRD) patterns were also recorded using a Rigaku (DMAX 2200 VPC) (Guangzhou, Guangdong, China) instrument equipped with CuKα radiation (λ = 1.5418 Å) for phase determination. In-situ variable-temperature PXRD (VT-PXRD) data were collected from 4 to 293 K (SmartLab, CuKα, λ = 1.5418 Å). Room-temperature synchrotron powder XRD (SPXRD) data were collected at ambient and high pressures on beamline BL14B (λ = 0.68840 Å) and BL15U1 (λ = 0.6199 Å), respectively, at the Shanghai Synchrotron Radiation Facility (SSRF). A symmetric diamond anvil cell (DAC) with culet sizes of 250 μm and a Re gasket was used. Silicon oil was used as pressure transmitting media (PTM), and the pressure was determined by the ruby luminescence method. 2D diffraction images were analyzed using FIT2D software. Refinements of the SPXRD and single-crystal diffraction data were performed using programs of Topas Academic V6 and Olex2 (1.2), respectively.32,33 Thermogravimetric analysis (TGA) was carried out using an NETZSCH TG 209F1 Libra analyzer (Guangzhou, Guangdong, China). Two copies of crystals with a mass of 4.89 mg were placed into two alumina crucibles and heated at a rate of 10 K min−1 to 1173 K in Ar atmosphere. Physical properties and spectroscopic measurements The magnetic properties were measured on a Magnetic Property Measurement System (MPMS3) over a temperature range of 3 and 380 K. The susceptibility was evaluated in both zero-field-cooled (ZFC) and field-cooled (FC) modes under applied fields of 0.1, 1, and 7 T. The specific heat was measured with a two-tau relaxation method in a Physical Property Measurement System (PPMS-9T). The temperature-dependent resistance of AgRuO3 under different pressures was measured using a DAC made of BeCu alloy ranging from 2.6 to 36.3 GPa. The diameter of the diamond culet was 300 μm. The composite gasket consists of rhenium and insulating c-BN, where KBr used as a PTM was loaded into the sample chamber (∼100 μm), in which a AgRuO3 sample with dimensions of ∼90 μm × 30 μm × 20 μm and a ruby chip were placed. Four pieces of thin platinum were utilized as electrical contacts. The pressure was calibrated at room temperature using the ruby fluorescent method. Temperature dependent resistance measurements in DAC were performed in a liquid-helium cryostat. The X-ray absorption near edge spectroscopy (XANES) data were collected in the total electron yield mode at the Brookhaven National Synchrotron Light Source (NSLS-II) on beamline 7-ID SSA-2 using a Si (111) double crystal monochromators. In situ variable pressure Raman experiments were performed on crystals using a Linkam THMS350V with an excitation wavelength of 532 nm. High pressure was generated using a DAC ranging from 1.07 to 20.20 GPa. Theoretical calculations We have explored the crystal structures of AgRuO3 by employing a particle swarm optimization algorithm as implemented in the CALYPSO code in conjunction with first-principles density functional theory (DFT) total-energy calculations.34–36 Based on the projector augmented-wave method, DFT was implemented in the Vienna Ab Initio Simulation Package.37–39 The plane wave cut-off energy and k-point were 700 eV and 9 × 9 × 9 for geometry optimization and static electronic calculations, respectively. The convergence factor was set as the difference in total energy within 1.0 × 10−7 eV per atom. All atomic positions were fully relaxed until the remaining force on each atom was less than 1.0 × 10−2 eV·Å−1. The exchange correlation energy was treated by the generalized gradient approximation Perdew–Burke–Ernzerhof (GGA-PBE).40 Electron-electron Coulomb repulsion interactions (U) for Ru 4d orbitals were considered in the rotationally invariant form (GGA+U) with Ueff = 2.0 eV as used to predict semiconducting behavior of AgRuO3.31,41 Results and Discussion Revisiting the crystal and electronic structures and magnetism at AP Thermal stability of the as-made AgRuO3 was examined by TGA analysis ( Supporting Information Figure S1), which shows that AgRuO3 is stable up to around 400 °C in Ar before decomposing into a mixture of Ag and RuO2 ( Supporting Information Figure S1b). The observed weight loss of 6.31% is in reasonable agreement with the expected value of 6.23% due to oxygen release. The crystal structure of AgRuO3 at room temperature was determined by SCXRD, which yielded a rhombohedral R-3 rather than the previously reported R-3c ( Supporting Information Table S1) at the experimental conditions.31 At 100 K, the unit-cell volume (381.70(3) Å3) is almost identical to that at room temperature (381.75(4) Å3) as shown in Supporting Information Table S1, owing to the slight expansion (0.005 Å) in the ab-plane and contraction (0.031 Å) along the c-axis upon cooling. The unit-cell parameters extracted from in situ variable temperature SCXRD reveal that AgRuO3 undergoes a steep NTE in the ab-plane and a soft positive thermal expansion along the c-axis, rendering a slight overall cell contraction between 100 K and 293 K. This phenomenon can also be observed in Supporting Information Figure S2, where no additional structural transition appears down to 4 K. The anomalous thermal expansion could be responsible for the weak phase transition or the crossover between 125 and 200 K reported in literature.30 The R-3 AgRuO3 adopts the typical ilmenite structure (Figures 1a and 1b, Supporting Information Figure S3, and Tables S1 and S2) with alternative RuO6 and AgO6 honeycomb layers in the ab-plane, where the face-shared AgO6 and RuO6 octahedral pair along the c-axis are obstructed by octahedral vacancies. The refined cell dimension (Figure 1b, R-3, a = 5.2251(6) Å) in the ab-plane is similar to that in the literature (Figure 1c, R-3c, a = 5.2261(6) Å), while the unit cell along the c-axis (c = 16.0580(1) Å) is about half of the reported value (c = 32.358(5) Å). The obtained contractible structure is reproducible on single crystals from different batches, which were measured on different diffractometers with either Ga-In or Mo X-ray targets ( Supporting Information Tables S1 and S2), so the possibility of defect-induced uncertainty of structural determination can be ruled out by both SCXRD and PXRD measurements ( Supporting Information Figure S4). High resolution SPXRD data fittings (Figure 1d, R-3, a = 5.2412(2), c = 16.0700(6), V = 382.31(3), Rp/Rwp = 6.37/9.18%), which, to a great extent, are more reliable than the single-crystal diffraction way, further corroborating the validity of our R-3 structure. First-principles calculations are expected to distinguish between the more energetically stable structure between R-3 and R-3c. According to the ground state of AgRuO3 with AFM1 and semiconducting behavior, we further calculated the corresponding energy of different ground states ( Supporting Information Table S3). Electron-electron Coulomb repulsion interactions (U) for Ru 4d orbitals were considered in the rotationally invariant form (GGA+U) with Ueff = 2.0 eV as used to predict the semiconducting behavior of AgRuO3.31 Based on the AFM1 ground state, R-3 demonstrated lower energy than that of R-3c (35 meV), indicating that the R-3 of AgRuO3 seems more stable. Combined with the experimental results, we can further conclude that AgRuO3 belongs to the R-3 space group at AP. Figure 1 | Crystal structure determination of AgRuO3. (a) The edge-sharing RuO6-honeycomb layer motif in the ab-plane projected along the [001] direction. (b) The polyhedral unit cell structure in R-3 symmetry in this work. (c) The reported supercell polyhedral unit cell structure in R-3c for comparison. (d) Le Bail fit of the SPXRD data in R-3 symmetry in this work. Download figure Download PowerPoint Monovalent Ag+ with a simple closed shell (d10) is the dominant oxidation state of silver. Compounds containing higher oxidation states of Ag are interesting but rare. Ag2+ (d9) in the fluoride was found to be stabilized by the most electronegative F element. In contrast, cuprate-resembling Ag2+ oxides are very rare beyond binary systems. AgBO3 (B = Nb, Ta, Sb, Bi), AgNbO3 (Cmcm, Pbcm, and Pm-3m), and AgTaO3 (R3cH and R-3mH) adopt perovskite-related or LiNbO3-type structures and span a range of different coordination environments from 6 to 12, while AgSbO3 (R-3H) and AgBiO3 (R-3H) take analogous structures with AgRuO3.42–45 There are three shorter and three longer Ag–O bond lengths within the AgO6 units in AgRuO3 (Figure 2a). A comparison of the Ag–O bond length in R-3 AgBO3 is shown in Figure 2b, where the Ag–O distance in AgRuO3 is obviously shorter than those in AgBiO3 and AgSbO3. Generally, shorter Ag–O bond lengths mean higher oxidation states of Ag considering its ionic radius (1.15, 0.79, and 0.75 Å for Ag+, Ag2+, and Ag3+, respectively, in octahedral coordination) at different valence states.46 The bond valence sum (BVS) calculations of Ag give somewhat larger values in AgRuO3 than those in AgSbO3 and AgBiO3 (Figure 2b). Considering the shorter distance between Ag-Ru (3.174(1) Å) within the face-shared octahedral pairs, the higher oxidation state Ag ion could be ascribed to non-negligible covalent bonding nature as the case in fluorides, or charge disproportion between Ag and Ru, which, together with the long distance (5.406 Å) between the honeycomb RuO6 layers, endows remarkable 2D features of AgRuO3. Figure 2 | Local environment and electronic structure. (a) The distorted AgO6 octahedron in AgRuO3 at AP. (b) Comparison of the Ag–O bond length and corresponding BVS of Ag in AgBO3 (B = Ru, Bi, Sb) at AP and room temperature; XANES of AgRuO3 for (c) Ag-L3 edge and (d) Ru-L3 edge. Download figure Download PowerPoint XANES data were further analyzed to confirm the oxidation state of cations. As displayed in Figure 2c, Ag L3-edge XANES clearly reflects an emerged peak located at higher energy in AgRuO3 compared with Ag+ in Ag2O, which is also distinct with Ag+ and Ag3+ in AgO (Ag2O2). Since the nominal oxidation state of Ru in AgRuO3 is between Ru4+ and Ru5+ referenced to Ru4+O2 and Ca2YRu5+O6 (Figure 2d), the charged nominal formula of the title compound can be thus written as Ag+1/+2Ru+4/+5O3. The mixed-valence case could also be found in the lighter homologue of Ag, such as the Cu1+/Cu2+ ions in Cu2IrO3.16 The electronic structure and valence state are also manifested by the magnetic properties, which are strongly related to the electronic configuration and exchange interactions. Ag2O, Ag2O3, and AgO are diamagnetic, which is expected for Ag+ (d10) and square-planar Ag3+ (d8) ions. Distinctly, the d9-Ag2+ could endow interesting magnetic properties, such as a one-dimensional chain of Ag2+ ions with strong AFM coupling in AgSO4 and a 2D AFM square net of Ag2+ ions in Ag(pyz)2(S2O8) (pyz = pyrazine).47,48 Compared with the discoveries in literature, the inconsistency of crystal and electronic structure motivated us to reexamine the magnetism of the as-made AgRuO3.30,31 The temperature-dependent magnetic susceptibility of polycrystalline AgRuO3 is nearly identical to that reported in literature, as shown in Supporting Information Figure S5. A sharp AFM transition is observed around 340 K in the FC curve at 0.1 T. The bifurcation of ZFC and FC curves appears upon cooling at 0.1 T below the AFM transition point. The magnetic susceptibility value is less magnetic field-dependent above 1 T and keeps decreasing upon cooling to 75 K, which is then followed by upturns in the χ(T) curves. At 7 T, the ZFC and FC curves nearly overlap. The presence of magnetic Ag2+ ions and interactions between Ag2+ and Ru4+/5+ could be responsible for the bump around 50 K and transition around 120 K in the 0.1 T-ZFC χ(T) curve in Supporting Information Figure S5a. Besides, these magnetic transitions in the 0.1 T curves are strongly dependent on the applied field and highly suppressed at higher magnetic field, suggesting complicated magnetic phase transitions. The magnetic-field dependent Cp(T) curves ( Supporting Information Figure S5b) were measured on a pellet (cold pressed at 3 GPa using the polycrystalline powder). The low-temperature Cp(T) data can be appropriately fitted by Cp(T) = γ'T + βT3 + δT5 (Cp/T − T2 plots in inset of Supporting Information Figures S5b), resulting in γ' = 5.1(6) mJ mol−1 K−2, β = 0.885(1) mJ mol−1 K−4, and δ = −0.00048(1) mJ mol−1 K−6, which are very comparable to those reported by Jansen et al.30 Taken together, the electronic structure of Ag and Ru is mixed d9/d10 and d3/d4, respectively, in AgRuO3. Temperature- and pressure-dependent resistance results Pressure is a unique thermodynamic variable to explore the phase transitions and novel physical phenomena inaccessible at ambient conditions.49,50 The resistance (R) of polycrystalline AgRuO3 was measured by a four-probe method in a DAC between 3 and 300 K up to 36 GPa ( Supporting Information Figure S6). As shown in Figure 3a, the temperature-dependent R(T) at 2.6 GPa exhibits a semiconducting behavior, similar to that at AP in literature.30R(T) monotonically decreases nearly four orders of magnitude as the pressure increased from 2.6 to 3.0 and then to 4.5 GPa. Although the R(T) at 3.0 and 4.5 GPa retains a semiconducting behavior, it becomes almost temperature-independent and close to the IMT boundary. Metallization appears from 4.5 to 8.7 GPa above 50 K, where R(T) initially decreases upon cooling and reaches a plateau between 50 and 30 K, followed by an upturn to semiconducting behavior at lower temperatures (Figure 3b). Similar metallic-to-semiconducting transitions were maintained at 12.3 GPa but showed lower R values and less temperature dependence. At 16.1 GPa, R(T) is slightly increased and becomes completely temperature-independent, approaching an unexpected crossover of MIT. The R(T) plots become upward warping in the whole temperature region upon cooling at incremental pressures ranging between 22.1 and 36.3 GPa, where the upturns with steeper slopes are observed below ∼20 K (Figure 3c). The overall R reaches a minimum (0.43 Ω) at 27.8 GPa, except the crossover with the 22.1 GPa plot below 75 K, where the R values of 22.1 GPa is slightly lower. At 36.3 GPa, R increases more rapidly upon cooling below 250 K. The R evolution can be more intuitively reflected by the P-T heat map in Supporting Information Figure S7. Figure 3 | Pressure dependent resistance variation between 3 and 300 K. (a–c) Pressure dependent R variation plots of AgRuO3 up to 36.3 GPa. (d) Pressure- and temperature-dependent heat map of R evolution of AgRuO3. Download figure Download PowerPoint The overall pressure- and temperature-dependent phase diagram of R is shown in Figure 3d. AgRuO3 undergoes a sequential IMT and MIT transition when pressing up to 8.7 and 22.1 GPa, respectively. It is commonly believed that the metallic state is favored at high pressures, where the pressure-induced IMT in AgRuO3 can be roughly attributed to enhanced band overlapping from lattice shrinkage.51–53 Nevertheless, pressure-induced MIT has also been discovered in several quantum materials, since enhanced structural distortion can weaken the electron hopping and lead to localization in an insulating state.54–56 The cell dimension keeps changing at increasing pressure, so it is impossible to generate the exact shape factor and thus, resistivity, for the detailed simulation of the conduction mechanism. Further, the upturns at lower temperatures need further investigation. The pressure-dependent isothermal R(P) curves are displayed in Supporting Information Figure S8. At high temperatures, R decreases with increasing pressure, while at low temperatures, R drops to a minimum around 12.5 GPa, and then starts to rise until ∼16 GPa, followed by less pressure-dependent concave curves at higher pressure. To correlate the pressure-dependent R evolution and structural modification in AgRuO3, in-situ pressure-dependent SPXRD and Raman spectra were conducted at room temperature with smaller pressure intervals. Pressure-induced intermetallic charge transfer To assess the origin of the successive IMT-MIT in AgRuO3, in situ pressure-dependent SPXRD patterns were collected at room temperature under various pressures to study the detailed structural modification (Figure 4a). The SPXRD pattern at 0.14 GPa can be well fitted by the ambient-pressure rhombohedral R-3 structure regardless of the absent peaks due to preferred orientation. Interestingly, as displayed in Figure 4b, anisotropic peak shifting is observed in the intermediate pressure (metallic) region, which is distinct to the cases at lower pressures. Up to 14.45 GPa, extra peaks appear in the patterns, indicating the emergence of pressure-induced structural phase transition. Remarkably, these structural changes are consistent with the successive MIT in the AgRuO3. Figures 4c and 4d display the pressure dependence of normalized lattice parameters deduced from Le Bail fit of the SPXRD patterns. Apparently, anisotropic compressibility arises in AgRuO3. As expected, the c-axis contracts with increasing pressure, indicating a narrowing of the interplanar distance. In contrast, the ab-plane exhibits robust anti-compressibility with negligible changes and an abnormal negative compressibility is observed in the pressure region where metallic behavior is observed, indicating that the structural modulation has a profound impact on the electrical transport property. Figure 4 | Pressure-induced structural evolution. (a) In-situ variable pressure SPXRD patterns collected at various pressures for AgRuO3 upon pressing, the colored dash-line arrows highlight the peak evolution upon pressing. (b) SPXRD patterns at selected pressure to illustrate the peak shift, and the azury rectangles at 14.45 GPa mark the new peaks. (c and d) Pressure dependences of the lattice parameters. Download figure Download PowerPoint Due to angular constraints and background from the DAC environment, the detailed crystallographic information cannot be decently refined from the SPXRD data. Combined with the deduced lattice parameters, CALYPSO search provides reasonable crystal information for the pressure range of the first IMT. Figure 5a displays the calculated pressure-dependent local structural evolution of the face-shared octahedral pair in the R-3 zone, where the Ag-Ru distance decreases with shrinkage along the c-axis, from 3.174 Å at AP to 2.993 Å at 14.45 GPa, which is indicative of strong orbital overlap between Ru and