High Temperature Structure and Thermal Expansion of Some Metals as Determined by X-Ray Diffraction Data. I. Platinum, Tantalum, Niobium, and Molybdenum

The crystal structure and thermal expansion of platinum, tantalum, niobium, and molybdenum have been determined between 1100° and 2500°K. These metals were found to undergo a uniform thermal expansion over the temperature range of this investigation and to undergo no structural change. The permanent elongation of tantalum wires, produced by annealing at temperatures between 2473° and 2773°K, appears to be caused by reorientation of crystal grains in the specimen and to preferred direction of crystal growth during annealing, rather than to a change in crystal structure. Quadratic equations have been developed for the thermal expansion of platinum, tantalum, niobium, and molybdenum. These equations are represented, respectively, by Δa0/a0=7.543×10−6(T−291)+2.362×10−9(T−291)2,Δa0/a0=6.080×10−6(T−291)+7.50×10−10(T−291)2,Δa0/a0=7.591×10−6(T−291)+6.96×10−10(T−291)2, and Δa0/a0=0.987×10−3+2.40×10−6(T−273)+2.20×10−9(T−273)2. Values of the expansion coefficient α were computed for each of the metals by differentiating the above equations. Our values were compared with those obtained from the Grueneisen theory and indicate that the theory provides a reasonably satisfactory means of extrapolating thermal expansion data obtained at low and moderate temperatures.