Concomitant Ordering in First Row Transition Metal Fluorides

Author

Ajay Shah

Date of Award

8-1-1990

Document Type

Dissertation

Degree Name

Ph.D. in Chemistry

Department

Chemistry and Biochemistry

First Advisor

W. O. J. Boo

Second Advisor

Andrew P. Stefani

Third Advisor

Walter E. Cleland

Fourth Advisor

Kwang S. Yun

Relational Format

dissertation/thesis

Abstract

Type of Compounds Investigated

First row transition metal fluorides with a general formula, A_xM²⁺_xM’³⁺_1-xF₃, where A = alkali metal ion, M,M' = First row transition metal ions, and x = 0 - 1 ate reported to form, ReO₃, perovskite, GdFeO₃, tetragonal tungsten bronze (TTB), hexagonal tungsten bronze (HTB), and, modified pyrochlore type structures. Compounds of the type LiM^IIM^IIIF₆ have been reported to form rutile and trirutile type structures. These compounds display ionic, electronic, and magnetic ordering.

Formation of TTB Structure and Linear Trimers

The TTB structure demonstrates ordering between M²⁺ and M³⁺ ions. Consequently, a variety of TTB fluorides have been tailored that display anitferro-, ferri- or ferro-magnetism. By substituting a diamagnetic ion for either M²⁺ or M³⁺, the diluted paramagnetic ions form linear trimers. These linear trimers are useful for the study of 180° M-F-M interactions. Structural and magnetic properties of several quaternary TTB fluorides are reported.

The Series KxCrF 3

The series K_xCrF₃ (x ≈ 0.4 - 0.6) was reported to form the hexagonal BaTa₂O₆ type structure. It was believed that the TTB structure did not form due to the Jahn-Teller ion Cr²⁺. The unit cell did not show any evidence of distortion. Using superior synthesis and characterization techniques, we report formation of three phase in the series K_xCrF₃ (x ≈ 0.42 - 0.58). In the lower range the structure is a hexagonal BaTa₂O₆ type. In the middle composition range the structure is an orthorhombic distortion of the hexagonal unit cell. At higher values of x, formation of a TTB structure is observed. As expected, the tetragonal unit cell is distorted to orthorhombic due to cooperative Jahn-Teller ordering. Magnetic data support these observations.

Derivation of Symmetry Trees

It is observed that the symmetry of a crystal is lowered as concomitant ordering sets in. This observation in conjunction with the Symmetry Principle has been used to derive Symmetry Trees (or, symmetry lowering diagrams) for the systems mentioned above. These Symmetry Trees experimentally confirm Landau and Lifshitz's contention that ordering reduces symmetry. They also provide experimental evidence for the Special Symmetry Evolution Principle, (the degree of symmetry of the state of an isolated system cannot decrease during evolution but either remains constant or increases). In several cases, where experimental determination of a unique space group is not feasible, the symmetry tree approach is implemented to determine the space group.

Comments

Uncorrected.

Recommended Citation

Shah, Ajay, "Concomitant Ordering in First Row Transition Metal Fluorides" (1990). Electronic Theses and Dissertations. 3217.
https://egrove.olemiss.edu/etd/3217

Accessibility Status

Searchable text

Concentration/Emphasis

Chemistry