Systematic Study on the Role of the Third Zn-Site Element in Zn_2−xMg_xP₂O₇ Showing Giant Negative Thermal Expansion

Abstract

For Mg-doped Zn₂P₂O₇, this systematic investigation of co-doping onto Zn sites has elucidated specific effects on negative thermal expansion (NTE). The low-cost and low-environmental-impact NTE material Zn_2−xMg_xP₂O₇ shows large NTE in a temperature range including room temperature for x = 0.4. Although Mg doping broadens the operating-temperature window, it remains several dozen degrees wide. Moreover, the total volume change related to NTE becomes less than that of the Zn₂P₂O₇ parent material. Findings obtained from this study demonstrate that co-doping of Mg and of another element onto the Zn site is effective for achieving simultaneous expansion of the operating-temperature window and maintenance of the volume change related to NTE. One illustrative case is that Zn_1.64Mg_0.30Al_0.06P₂O₇ has a large negative coefficient of linear thermal expansion of about −65 ppm/K at temperatures of 300–375 K. In fact, at temperatures high above room temperature, Zn_1.64Mg_0.30Al_0.06P₂O₇ powder shows better thermal expansion compensation capability than the composition without Al. The Al-doped phosphates are expected to have broad practical application because of their performance, cost, and environmental load characteristics.

Co-doping of Mg and of Al onto the Zn site is effective for achieving simultaneous expansion of the operating-temperature window and maintenance of the volume change related to negative thermal expansion.