A central mount, back reflection, x-ray powder camera was designed for measuring thermal expansions. The camera is 16 cms. in diameter, has a temperature range from 0° to 1000° C., is inexpensive to construct and is simple to operate.

The linear (αa) and volume (αv) thermal expansion coefficients of ThO2 are αa = 0.6216×10–5+3,541×10–9T–0.1125T–2 and αa = 1.865×10–5+10.96×10–9T–0.3375T–2 where T is the temperature in degrees Kelvin from 298° to 1073° K. For periclase, the coefficients are are αv = 1.411×10–5+1.210×10–9T–0.5854T–2 and αv=4.232×10–5+3.630×10–9T–1.756T–2, from 298° to 973° K. For commercial diamond dust the coefficients are αv = 0.07667×10–5+3.672×10–9T–0.0903T–2 and αv = 0.2301×10–5+11.01×10–9T–0.2708T–2, and for a single crystal of South West African diamond, αa = 0.09613×10–5+3.522×10–9T–0.08882T–2 and αv = 0.2884×10–5+10.57×10–9T–0.2665T–2 from 298° to 973° K.

The measured expansions of periclase and diamond are slightly lower than previous measurements made on macroscopic single crystals. The thermal expansion of thoria has not been measured previously.

The unit cell size (a0) and molar volume (Vm) at 298.16° K. and 1 atmosphere are, for thoria, a0=5.59525±0.0001 Å and Vm=26.378±0.005 cc./mole; for periclase, a0=4.2117±0.0002 Å and Vm = 11.250±0.007 cc./mole; for diamond, a0=3.56688±0.00009 Å and Vm=3.4168±0.0001 cc./mole.

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