Thermodynamic studies of disorder in organic crystalline solids

Thesis

A calorimeter was constructed for the determination of heat capacities of solids from 1.5 to 84 K. Results from this and other calorimeters are discussed, on bis(adiponitrile)copper(I) nitrate and tetramethylammonium trichloromanganate(II) ("TMMC") from 1.5 to 300 K, and on β-modification metal-free, copper(II), and nickel(II) phthalocyanines from under 5 to 80 K.

Bis(adiponitrile)copper(I) nitrate has not-very-pronounced heat capacity maxima at 51 and 63 K. The total molar anomalous entropy change is estimated as between 1/2 R1n2 and R1n2. A previous assertion by X-ray crystallographers (Bull.Chem.Soc. Japan, 32, 1221 (1959)) that the nitrate ions are disordered among four orientations at room temperature is not supported by these results or by consideration of structure factors or of the potential field on the nitrate ions.

The TMMC heat capacity results are consistent with those given in Solid State Comm., 15, 1185 (1974) and Phys.Rev. B, 12, 5858 (1975). The anomalous molar entropy change associated with the monoclinic–hexagonal transition at 126 K is estimated as R1n2.8, of which 1/3 occurs between 124 and 127 K; it is suggested that the NC₄ tetrahedra undergo twofold disordering and that the H atoms also undergo some disordering.

The heat capacities of the copper and nickel phthalocyanines are surprisingly different from each other, especially below 25 K, where the former is markedly lower except for two first-order transitions at about 9–13 K with a total molar entropy change of not less than approximately R1n4. Magnetic and Schottky anomalies are, apparently, not involved in either material; perhaps the transitions in copper phthalocyanine involve disordering of the copper ions among four positions in the plane of each molecule. Below 8 K, the heat capacities of copper and metal-free (but not nickel) phthalocyanine apparently obey the T³ law.

Authors: Jewess, M; Jewess, Michael

• Publication date: 1978
• DOI: 10.5287/ora-2zjppypqw
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Thermodynamics; Order-disorder models; Crystals; Defects