KCo1/2Hf1/2(MoO4)2 – A Novel Glaserite-like Molybdate Exhibiting Two-dimensional Negative Thermal Expansion

Introduction Due to the peculiarities of the crystal structure and composition, complex molybdates exhibit valuable functional properties. Although the intensive search for NTE materials has been going on for many years, the investigation of the mechanisms and structure-property relationships causing the “thermal shrinkage” effect is still relevant. Here, we report on the 2D-NTE effect in a new ternary molybdate KCo 1/2 Hf 1/2 (MoO 4 ) 2 with a layered glaserite-like structure, and its thermal deformations and ionic conductivity were investigated. Methods The KCo 1/2 Hf 1/2 (MoO 4 ) 2 was synthesized through the ceramic method. HT-PXRD patterns were acquired using a Bruker D8 ADVANCE diffractometer and subsequently utilized for Rietveld analysis. Structure refinement and lattice parameter calculations were conducted with TOPAS 4.2, while calculation of the thermal expansion tensor and visualization were carried out in the TTT. ATR-IR spectroscopy was used to characterize the coordination of the molybdenum atoms. TG and DSC analyses were performed using an STA 449 F1 Jupiter thermal analyzer. Electrical conductivity was assessed using a Z-1500J impedance meter using the two-contact impedance spectroscopy method. Results KCo 1/2 Hf 1/2 (MoO 4 ) 2 crystallizes with the trigonal glaserite-type structure in space group P 3- m 1 with unit cell parameters of a = 5.76190 (3), c = 7.14788 (5) Å, V = 205.51 (1) Å 3 , and Z = 1. DSC indicated that KCo 1/2 Hf 1/2 (MoO 4 ) 2 melted at 699°C with decomposition. The observed 2D negative thermal expansion in the ab plane did not lead to a reduction in the cell volume. The studied ternary molybdate KCo 1/2 Hf 1/2 (MoO 4 ) 2 (α V = 148×10 –6 °С –1 at 600°С) can be categorized as a high-expansion material. Conclusion KCo 1/2 Hf 1/2 (MoO 4 ) 2 has been categorized as a high-expansion material (αV = 148 × 10− 6 ◦С− 1 at 600◦С). At higher temperatures, the compound demonstrated notable ionic conductivity, values up to 0.42·10 –3 S/cm (570°C) at Е а = 0.8 eV.