Synthesis of ZrSiO_4 and Coesite in SiO_2-ZrO_2 System Under High Pressure

ZrSiO4 and coesite were obtained under high-pressure and high-temperature from the nano precursor of a-SiO2 and ZrO2. XRD and Raman measurements indicate that ZrSiO4 was formed at a temperature higher than 920 ℃ under a pressure of 3.6 GPa. As the pressure increased to 3.9 GPa, the ZrSiO4 formation temperature was reduced to 815 ℃. The formation temperature for coesite was 990 ℃ under 3.9 GPa. The lower formation temperature for ZrSiO4, as compared to that for coesite, provided an experimental evidence that the coesite in the Earth＇s surface usually occurs as inclusions in ZrSiO4.

Synthesis and High Pressure Study on Ca_(1-x)Ln_xSiO_3

In this paper, for the first time, Ca1-xLnxSiO3 (Ln=Y. La, x=0.1, 0.2) was synthesized with three methods of high temperature (HT) under atmospheric pressure (AP), high temperature (HT) combined with high pressure (HP) and sol-gel method. The sample obtained at the stable range of 4.3-5.0 GPa pressure and 1050-1100℃ temperature with HT and HP method has a cubic perovskite structure, which has the minimum lattice parameter of 0.3535 nm so far,the sample with triclinic structure was obtained by sintering at atmosphere, and the sample synthesized with sol-gel method has monoclinic structure. SEM analysis shows that rare earth ions take the lattice site of Ca, but the distribution of rare earth ions in the samples synthesized with HT and HP is not homogeneous.

Magnetocaloric effect study of SrFe_(0.8)Co_(0.2)O_3 single crystal prepared under high pressure

A high-quality SrFe0.8Co0.2O3 single crystal is prepared by combining floating-zone and high-pressure treatment methods. Its Magnetocaloric effect is investigated by magnetic measurements. A paramagnetism-to-ferromagnetism tran- sition is found at about 270 K and this transition is a second-order one in nature as confirmed by Arrott plots. The saturated moment obtained at 2 K and 7 T is 3.63 μB/f.u. The maximal value of magnetic entropy change measured at 5 T is about 4.0 J·kg-1 ·K-1. The full wide at half maximum for a magnetic entropy change peak observed in SrFe0.8Co0.2O3 is considerably large. As a consequence, the relative cooling power value of SrFe0.8Co0.2O3 obtained at 5 T is 331 J/kg, which is greatly higher than those observed in other perovskite oxides. The present work therefore provides a promising candidate for magnetic refrigeration near room temperature.

High-pressure scheelite-type polymorph of SmCrO_4: synthesis, structural characterization and magnetic properties

The new scheelite form of SmCrO4 oxide was obtained by heating the zircon-type SmCrO4 oxide at 4 GPa and 803 K. X-ray diffraction revealed that this scheelite SmCrO4 phase crystallized with tetragonal symmetry, S.G. I41/a and lattice parameters: a=0.50776(3) nm and c=1.15606(2) nm. This structural phase transition from zircon to scheelite involved a decreasing of around 10% in the unit cell volume. Although the Cr-O and Sm-O distances did not change very much in both zircon and scheelite polymorphs, the changes occurred in the bond angles were remarkable that appear to support the proposed reconstructive model to explain this structural zircon-scheelite phase transition. Magnetic susceptibility and magnetization measurements revealed that the scheelite SmCrO4 oxide behaved an antiferromagnetic material, where the Sm3+ and Cr5+ were simultaneously ordered. The estimated Néel temperature, TN, was 16 K and the critical field at 12 K associated with the metamagnetic transition was 3.2 T.

High-pressure synthesis,crystal structure and physical properties of a new Cr-based arsenide La3CrAs5

In La-Cr-As system,the first ternary compound La3CrAs5 has been successfully synthesized under highpressure and high-temperature conditions.La3CrAs5 crystallizes into a hexagonal Hf5Sn3Cu-anti type structure with a space group of P63/mcm(No.193)and lattice parameters of a=b=8.9845A and c=5.8897A.The structure contains facesharing octahedral CrAs6 chains along the c-axis,which are arranged triangularly in the ab-plane and separated by a significantly large distance of 8.9845A.The magnetic properties,resistivity and specific heat measurements were performed.La3CrAs5 exhibits a metallic state with Fermi liquid behavior at low temperatures and undergoes a ferromagnetic transition at Curie temperature TC^50 K.First-principles theoretical studies were conducted to calculate its band structure and density of states(DOS),which indicated that the non-negligible contribution of La to the DOS near the Fermi level caused La3CrAs5 to be a three-dimensional(3D)metal.The crystal orbital Hamilton population(-COHP)was also calculated to explain the global stability and bonding characteristics in the structure of La3CrAs5.

Enhanced thermoelectric performance of Na-doped Pb Te synthesized under high pressure

Despite an effective p-type dopant for PbTe, the low solubility of Na limits the fully optimization of thermoelectric properties of Na-doped PbTe. In this work, Na-doped PbTe was synthesized under high pressure. The formation of the desired rocksalt phase with substantially increased Na content leads to a high carrier concentration of 3.2×10^20 cm^-3 for Na0.03Pb0.97Te. Moreover, dense in-grain dislocations are identified from the microstructure analysis. Benefited from the improved power factor and greatly suppressed lattice thermal conductivity, the maximal ZT of 1.7 is achieved in the optimal Na0.03Pb0.97Te. Current work thus designates the advantage of high pressure in synthesizing PbTe-based thermoelectric materials.

A-site ordered perovskite CaCu3Cu2Ir2O(12-δ) with square-planar and octahedral coordinated Cu ions

A novel CaCu_3Cu_2Ir_2O_（12-δ） polycrystalline sample was synthesized at 8 GPa and 1373 K.Rietveld structural analysis shows that this compound crystallizes in an AA＇_3B_4O_（12）-type A-site ordered perovskite structure with space group Im-3.Xray absorption spectra reveal a ＋2-charge state for both the square-planar and octahedral coordinated Cu ions,and the valence state of Ir is found to be about ＋5.Although the A-site Ca and the A＇-site Cu^（2＋） are 1：3 ordered at fixed atomic positions,the distribution of B-site Cu^（2＋） and Ir^（5＋） is disorderly.As a result,no long-range magnetic ordering is observed at temperatures down to 2 K.Electrical transport and heat capacity measurements demonstrate itinerant electronic behavior.The crystal structure is stable with pressure up to 35.7 GPa at room temperature.

Formation of unusual Cr^5+charge state in CaCr0.5Fe0.5O3 perovskite

A new oxide CaCr0.5Fe0.5O3 was prepared under high pressure and temperature conditions. It crystallizes in a B-site disordered Pbnm perovskite structure. The charge combination is determined to be Cr^5＋/Fe^3＋ with the presence of unusual Cr^5＋ state in octahedral coordination, although Cr^4＋ and Fe^4＋ occur in the related perovskites CaCrO3 and CaFeO3. The randomly distributed Cr^5＋ and Fe^3＋ spins lead to short-range ferromagnetic coupling, whereas an antiferromagnetic phase transition takes place near 50 K due to the Fe^3＋-O-Fe^3＋ interaction. In spite of the B-site Cr^5＋/Fe^3＋ disorder, the compound exhibits electrical insulating behavior. First-principles calculations further demonstrate the formation of CaCr0.5^5＋Fe0.5O3 charge combination, and the electron correlation effect of Fe^3＋ plays an important role for the insulting ground state. CaCr0.5Fe0.5O3 provides the first Cr^5＋ perovskite system with octahedral coordination, opening a new avenue to explore novel transition-metal oxides with exotic charge states.

Enhanced thermoelectric performance of Sb-doped Mg_(2)Si_(0.4)Sn_(0.6)via doping,alloying and nanoprecipitation

With the advantages of eco-friendliness,low cost,and low density,Mg_(2)(Si,Sn)solid solutions are promising candidates for thermoelectric applications.In this work,Sb-doped Mg_(2)Si_(0.4)Sn_(0.6)bulks were prepared with a combined method of solid-state reaction and high pressure synthesis,followed by spark plasma sintering.Our investigations show that Sb doping optimizes the carrier concentration,while Si/Sn alloying effectively suppresses the lattice thermal conductivity and induces a convergence of the two lowest-lying conduction bands.Additionally,numerous coherent Sn-rich nanoprecipitates are formed within micron-sized grains.All these factors contribute synergistically to improving the thermoelectric properties of Mg_(2)Si_(0.4)Sn_(0.6).The optimal Mg_(2)(Si_(0.4)Sn_(0.6))_(0.985)Sb_(0.015)exhibits a power factor higher than 4000 mW·m^(-1)·K^(-2)and a lattice thermal conductivity less than 0.8 W·m^(-1)·K^(-1)at temperatures higher than 600 K,leading to the highest ZT of 1.61 at 823 K.Current work demonstrates an effective approach to enhancing the thermoelectric performance of n-type Mg_(2)X solid solutions through doping,alloying,and microstructure modification.

Superconductivity above 70 K observed in lutetium polyhydrides

The binary polyhydrides of heavy rare earth lutetium that shares a similar valence electron configuration to lanthanum have been experimentally discovered to be superconductive.The lutetium polyhydrides were successfully synthesized at high pressure and high temperature conditions using a diamond anvil cell in combinations with the in-situ high pressure laser heating technique.The resistance measurements as a function of temperature were performed at the same pressure of synthesis in order to study the transitions of superconductivity(SC).The superconducting transition with a maximum onset temperature(Tc)71 K was observed at pressure of 218 GPa in the experiments.The Tcdecreased to 65 K when pressure was at 181 GPa.From the evolution of SC at applied magnetic fields,the upper critical field at zero temperatureμ0Hc2(0)was obtained to be~36 T.The in-situ high pressure X-ray diffraction experiments imply that the high TcSC should arise from the Lu4H23phase with Pm3n symmetry that forms a new type of hydrogen cage framework different from those reported for previous light rare earth polyhydride superconductors.