High-temperature elasticity of magnesioferrite spinel

The elastic moduli of magnesioferrite spinel, MgFe₂ O₄, and their temperature dependence have been determined for the first time by ultrasonic measurements on a polycrystalline specimen. The measurements were carried out at 300 MPa and to 700̊C in a gas-medium high-pressure apparatus. On heating, both the elastic bulk (K_S) and shear (G) moduli decrease linearly to 350̊C. By combining with extant thermal-expansion data, the values for the room-temperature K_S and G, and their temperature derivatives are as follows: K₀ = 176.3(7) GPa, G₀ = 80.1(2) GPa, (∂K_S∂T)_P = -0.032(3) GPa K^-1 and (∂G/∂T _P = -0.012(1) GPa K^-1. Between 350 and 400̊C, there are abrupt increases of 1.4% in both of the elastic moduli; these closely coincide with the magnetic Curie transition that was observed by thermal analyses at about 360̊C.