An Alumina Standard Reference Material for Resonance Frequency and Dynamic Elastic Moduli Measurement I. For Use at 25 °C.

Bars of sintered polycrystalline alumina were machined to nominal dimensions of 12.7 by 1.27 by 0.

Jump Rates for Point Defects in Special Positions Held by a Trapping Center of Noncubic Symmetry.

In a previous paper [J. Phys. Chem.

Surface-Layer Relaxation in the Dielectric Spectrum of CaF Doped With GdF.

CaF crystals doped with 0.1 percent GdF were observed to develop surface layers when annealed above 700 °C in air, during application of Pt paste electrodes. The conductivity in these surface layers was much higher than in the bulk, due presumably to the large numbers of anion vacancies produced by dissolved oxygen.

Orientation of Calcium Molybdate (CaMoO) and Other Single Crystals Having the Scheelite Structure.

Values of interplanar angles useful in distinguishing the [100] from the [110] direction in CaMoO are given to facilitate orientation by analysis of back-reflection Laue patterns. The distinction between the [001] and directions in the scheelite structure is discussed and examples of physical properties dependent upon this distinction are given. The [001] direction is denned in terms of the crystal structure and a procedure for identifying [001] using Laue patterns is described.

Splitting of Equivalent Points in Noncentrosymmetric Space Groups Into Subsets Under Homogeneous Stress.

Splitting of general positions in crystals into subsets of equivalent sites under homogeneous stress has previously been given for all centrosymmetric space groups; the tabulation is here completed for all space groups by listing the results for noncentrosymmetric space groups.

Internal Friction in ZrO Containing CaO.

Internal friction was measured on polycrystalline ZrO containing 4, 7, 10, 13, 16, and 20 mole percent CaO. In the range 10 to 20 percent a symmetrical internal friction peak occurs with its maximum at about 300 °C at 1 kHz; below 10 percent a nonsymmetrical peak occurs at somewhat higher temperature which may be associated with the presence of two phases. The symmetrical peak in as-sintered specimens has the same dependence on CaO content as that reported for electrical resistivity at 1000 °C and is probably caused by motion of oxygen vacancies.

Reduction of Crystallographic Point Groups to Subgroups by Homogeneous Stress.

It is assumed that the symmetry operations possessed by a homogeneously stressed crystal will be those common to the crystal and to the stress. Application of stress either leaves a point group unaltered or lowers it to a subgroup. Any stress-induced minimum step of symmetry lowering can be caused by unaxial stress but three of the possible stress-induced compound steps of symmetry lowering require biaxial stress.

Splitting of a Set of Equivalent Sites in Centrosymmetric Space Groups Into Subsets Under Homogeneous Stress.

It is assumed that the symmetry operations possessed by a homogeneously stressed crystal will be those common to the crystal and to the macroscopic state of stress. Application of stress either leaves a space group unaltered or lowers it to a subgroup. Such lowering can always be considered to take place in successive steps each of which leaves no group intermediate between the starting group and the subgroup.

Some Elastic Compliances of Single Crystal Rutile From 25 to 1000 °C.

Young's modulus, as a function of temperature up to 1000 °C, was determined for eight rutile specimens of different crystallographic orientations. From these, the following four elastic compliances or combinations of them were determined as a function of temperature, 8 , 2S + S, and s - S - S/2. The method used was the same as had been used in a previous study for rutile at room temperature.

Relaxation Modes of Trapped Crystal Point Defects: the Three-Neighbor Shells Model in NaCl.

The results of a relaxation-mode analysis are presented for two cases of trapped-defect relaxation in the NaCl structure, in which both defects occupy the same type of site (e.g., impurity divalent ion and trapped vacancy), or in which they occupy the two different types of sites (vacancy pair).

Reduction of Space Groups to Subgroups by Homogeneous Strain.

It is assumed that the symmetry elements possessed by a strained crystal will be those common to the unstrained crystal and to the macroscopic state of strain. This principle has been applied to show all of the possible subgroups to which a given space group can be lowered by homogeneous strain for all of the 230 crystallographic space groups.

Symmetry Splitting of Equivalent Sites in Oxide Crystals and Related Mechanical Effects.

Changes in the symmetry of a crystal caused by an applied strain have been used to show in what circumstances an internal friction peak can result from the motion of isolated point defects. General rules are given to make the prediction, and these are applied to several structures of common oxides. The prediction for rutile is compared with experimental results which are interpreted by the movement of titanium ions between interstitial positions in the structure.

A Method for Determining the Elastic Constants of a Cubic Crystal from Velocity Measurements in a Single Arbitrary Direction; Application to SrTiO.

Three independent velocities of sound can be measured along any direction of propagation in a cubic crystal except the [100] and [111] directions. These three velocities suffice to determine the three elastic constants and for the [110] direction, the calculation of these constants is easy. For all other directions, the calculation is more difficult; the only existing method appears to be a perturbation technique developed by Neighbours.

Elastic Constants of Synthetic Single Crystal Corundum at Room Temperature.

The six elastic constants (and six elastic compliances) of corundum were determined in the kilocycle per second frequency range by an accurate resonance method. The results were checked in the megacycle per second range with a less accurate, pulse velocity method. The elastic moduli for polycrystalline alumina calculated from the single crystal compliances determined by the resonance method are in good agreement with experimental values obtained on high density polycrystalline alumina.