Thermodynamic Characteristics of the Sodium Dimolybdate Single Crystal: Enthalpy and Heat Capacity

The standard enthalpy of formation of sodium dimolybdate was determined by dissolution calorimetry based on the measured enthalpies of dissolution of Na 2 CO 3 , MoO 3 , and Na 2 Mo 2 O 7 in a 0.2 M NaOH solution and the literature data: Δ f H 0 (Na 2 Mo 2 O 7 , 298.15 K) = −2245.3 ± 6.3 kJ/mol. The lattice enthalpy was calculated using the Born–Haber cycle: –54 730 kJ/mol. The wavelength of luminescence radiation decreased from 650 to 540 nm on passing from sodium molybdate to sodium tungstate, and the lattice enthalpy decreased from –54 730 kJ/mol (Na 2 Mo 2 O 7 ) to –49 030 kJ/mol (Na 2 W 2 O 7 ). The temperature dependence of the heat capacity of sodium dimolybdate was determined in the temperature range 320–785 K. There are no phase transitions in this range.