Computer modelling of thorium doping in LiCaAlF6 and LiSrAlF6: application to the development of solid state optical frequency devices
This paper describes computer modelling of thorium doping in crystalline LiCaAlF6 and LiSrAlF6. The study has been motivated by the interest in using these materials as hosts for 229Th nuclei, which are being investigated for use as frequency standards. The dopant sites and form of charge compensation are obtained; this information is essential for the further development and optimization of these devices.
GULP: A computer program for the symmetry-adapted simulation of solids
Algorithms for the symmetry-adapted energy minimisation of solids using analytical first and second derivatives have been devised and implemented in a new computer program GULP. These new methods are found to lead to an improvement in computational efficiency of up to an order of magnitude over the standard algorithm, which takes no account of symmetry, the largest improvement being obtained from the use of symmetry in the generation of the hessian. Accelerated convergence techniques for the dispersion energy are found to be beneficial in improving the precision at little extra computational cost, particularly when a one centre decomposition is possible or the Ewald sum weighting towards real-space is increased.
The Application of the Three‐Dimensional Patterson Method and the Crystal Structures of Proustite, Ag3AsS3, and Pyrargyrite, Ag3SbS3
It is shown that the three‐dimensional Patterson method can be so simplified by the use of the symmetry properties of the crystal under consideration that its use in determining the positions of atoms in crystals is practicable. This method is then used to determine the positions of the heavy atoms in proustite, Ag3AsS3. The positions of the sulfur atoms are found by the use of the covalent atomic radii and assumptions concerning bond directions. The structure so derived is found to be compatible with the observed data. Proustite is found to contain pyramidal AsS3′′′ groups and silver atoms forming two bonds to sulfur in almost opposed directions. Pyrargyrite, Ag3SbS3, is found to have almost the same structure as proustite. The analytical statement of the structures is as follows:— The space group, C3v6 — R3c, is common to both proustite and pyrargyrite. The special positions of C3v6 are (in hexagonal axes) (2a)0,0,Z;0,0,12+Z;(6b)X,Y,Z;Y¯,X−Y,Z;Y−X,X¯,Z;Y¯,X¯,12+Z;X,X−Y,12+Z;Y−X,Y,12+Z; and positions der...