Adiabatic heat capacity and related properties of ammonium hexachlororuthenate, (NH4)2RuCl6, at temperatures from 5 ⩽ T/K ⩽ 325

• Heat capacity of solid (NH4)2RuCl6 from 5 K to 325 K using adiabatic calorimetry.
• One thermal anomaly in the salt identified.
• Thermodynamic functions derived over a wide range of temperature.
• Crystal structure of (NH4)2RuCl6 confirmed to be non-cubic.

The heat capacity of ammonium hexachlororuthenate, (NH4)2RuCl6, was measured over the temperature range 5 ⩽ T/K ⩽ 325 using adiabatic calorimetry. Our results show that a structural phase transition is not present. However, an anomalous increase in the heat capacity Cp,m occurs in the region 70 ⩽ T/K ⩽ 105 and the heat capacity reaches its maximum at T = (93.5 ± 0.1) K. The molar entropy change associated with this increase amounts to ΔSm = (0.465 ± 0.006) R. Standard molar thermodynamic functions are presented at selected temperatures from 5 K to 325 K.