Synthesis, Structure And Mechanism Of Low Thermal Expansion Oxide Ceramics （Ta,Nb）₂WO₈ And K₂Zr₂P₂SiO₁₂

The sharp changes in thermal expansion of high precision fields such as spacecraft,precision electronic devices,astronomical telescope mirror,often greatly affect the accuracy and service life.An effective way is to design single-phase compounds with zero/low thermal expansion over wide temperature region.However,such compounds are few and the related physical mechanism is not clear.Therefore,based on the framework structure flexibility,this work facus on exploring new low thermal expansion material with wide temperature region,revealing its thermal expansion mechanism and investigating other physical properties.A single-phase zero/low thermal expansion material Ta₂WO₈(α_l=-1.69×10^-6 K^-1,300-1000 K)was synthesized by solid-state sintering.It has no water absorption and phase transition in this temperature range.Ta₂WO₈ has orthogonal structure（Pbcm）.The Ta1O₇ decahedron is connected with two Ta2O₆,two Ta3O₆,and one Ta4O₆octahedrons by edges,and the six polyhedrons form a repeating unit along the bc plane.Along the a axis,the O atom is located between two identical Ta atoms and forms a Ta-O-Ta atomic linkage.The rhombic tunnels formed by the Ta3O₆ octahedron and the surrounding polyhedron have larger space and flexibility and are benefit to the transverse vibration of oxygen atoms.Therefore,the contraction of the Ta3-O9-Ta3bond angle is more obvious.The zero/low thermal expansion comes from the balance effect of negative thermal expansion on a axis,positive thermal expansion on b axis and c axis.The variable pressure Raman results show that the most low-frequency lattice modes related to translational motions of oxygen atom in the Ta-O-Ta linkage with negative Grüneisen parameters,which plays a key role in the anomalous thermal expansion of Ta₂WO₈.The results of UV-vis diffuse reflectance spectroscopy and density functional theory（DFT）show that Ta₂WO₈ has semiconductor properties.The Nb₂WO₈ ceramic material was synthesized by solid-state sintering.It exhibits linear low thermal expansion(α_l=2.30×10^-6 K^-1)from 173 to1073 K with no water absorption or phase transformation.Nb₂WO₈ has an orthogonal structure（Pbcm）.The Nb4O₇ decahedron is connected with two Nb1O₆,two Nb2O₆,and one Nb3O₆octahedrons by edges,and the six polyhedrons form a repeating unit along the bc plane.Along the a axis,the O atom is located between two identical Nb atoms and forms a Nb-O-Nb atomic linkage.The Nb2-O2-Nb2 bond angle decreases during the heating process,resulting in the NTE degree of a axis is not as severe as the PTE degree of b axis and c axis,thus the bulk phase exhibits low thermal expansion characteristics.The variable pressure Raman shows that the translational and rotational modes of the polyhedron play a key role in low thermal expansion.The UV-vis diffuse reflectance spectroscopy shows that Nb₂WO₈ is a semiconductor with a band gap of 2.66 e V.A low thermal expansion NZP system material K₂Zr₂P₂Si O₁₂ was prepared by ball-milling and solid-state method.It exhibits low thermal expansion(α_l=2.43×10^-6K^-1)in the temperature range of 300 to 1073 K with no water absorption and phase transformation.The variable temperature Raman results show that the Raman peak at111 cm^-1 has blue shift,indicating that the translational and libration of PO₄ tetrahedron and Zr O₆ octahedron contribute to its low thermal expansion.In addition,the resistivity of K₂Zr₂P₂Si O₁₂ decreased with the increase of temperature and its resistivity was 2000Ω·m at 1100 K.