Relationship Between Atomic Structure And Selected Physical Properties Of Transparent MGAL204

Abstract

ABSTRACT To better understand the basic solid state mechanism affecting select physical proper­ ties, structural measurements were made using a relatively new Nuclear Magnetic Resonance (NMR) technique. Historically, NMR has proved invaluable to the organic researcher for determining molecular structure (s) . For about a decade, exciting work has been ongoing which has developed and continually refined the technique for solid state NMR. A family of ceramics known as spinels has a natural crystalline structure which is well defined. Synthetic spinels do not always adhere to the same strict coordination site scheme. Initial NMR measurements seem to indicate a possible correlation between the measured coordination site location (s) of cations and select physical properties. 1 . INTRODUCTION The continuing need for hard, strong, broad transmission bandwidth materials has been met in recent years by, not a new glass or plastic, but a polycrystalline ceramic. This material is a specially processed Magnesia-Aluminate spinel (MgAl204) . Spinels do occur naturally or, as in the case of this optical ceramic, are synthesized. Spinels as a family of materials fall into two structural groups, and inverse. The MgAl204 is a spinel in theory and the A1+++ ion is found in octahedral coordination while the Mg++ is all in Tetrahedral Coordination. An spinel would have a theoretical structure where all the Mg++ are coordinated octahedrally while half the A1+++ fill the remaining octahedral sites and the other half go into tetrahedral locations.It has been mentioned in the literature, that while most naturally occurring spinels adhere to the theoretic structural scheme, synthetic versions may not. Through recent advances in NMR techniques, this paper strives to examine relationship between the measured structure of an optical quality MgAl204 and select physical properties of the ceramic. The structural determination was done at the regional NMR center at Colorado State University, Fort Collins, Colorado.2. SPINEL STRUCTURE 2 . 1 Theoretical Structure ( s )The polymorphic classification known as spinels encompasses a reasonably large variety of material. All spinels have the following formula:A++B2+++04The so called normal spinels are face centered cubic unit cells with all trivalent cations (B+++) in octahedral sites and divalent (A++) in tetrahedral sites. It must be pointed out, that there also exists an inverse spinel; in this structure, which is also cubic, the divalent cations occupy the octahedral sites while the trivalent cations fill the remaining octahedral sites with the balance going into tetrahedral coordination with the oxygens 1.2.1.1 MgAlpQa. Naturally occuring Magnesia-Aluminate spinel (MgAl204) occurs as a spinel structure. This is to say that all Mg++ ions should be in tetrahedral coordination with the oxygens and all A1+++ should be in octahedral sites.2.1.2 Synthesis History. A reasonable amount of work has been reported relating to the laboratory (or industrial) synthesis of MgAloOa powders 2-4 ^ ^ further step has been taken in the literature; and work is reported?-? , that uses NMR measurements to determine that synthesized, supposedly normal spinels do indeed have non-stoichiometric coordina­ tion site ratios. General results reported, indicate that synthesized (as opposed to natural) MgAl204 powders do not strictly adhere to the proper theoretical coordination site versus cation charge stoichiometrics as defined for spinels.2.2 NMR Measurements from the Regional Lab. To date, NRM measurements* have been made*NMR spectras were obtained on a Bruker AM-600 NMR spectrometer CH-1 resonance frequency of 600 Mhz . All work was done by the Colorado State Regional NMR Center, funded by National Science Foundation Grant No. CHE-8816437.