The Crystal Structures of Some Rhombic Formates (Continued Report III) Barium and Lead Formates, App.

Abstract

Using the method of analysis same as that described in the previous two reports2)3) on calcium and strontium formates, the crystal structure of barium and lead formates was determined. Both crystals correspond to the same space group D42-P212121as strontium formate. The dimensions of the unit cell containing four chemical units are: a=6.80 kX, b=8.89 kX, c= 7.66 kX for Ba (HCO2) 2, a=6.52 kX, b=8.75 kX, c=7.41 kX for Pb (HCO2) 2. All the atoms occupy sets of general positions : xyz; 1/2+x, 1/2-y, -z; 1/2-x, 1/2+y, 1/2-z; -x, -y, 1/2+z. By means of the syntheses of the two-dimensional Patterson functions and electron densities, the atomic parameters were obtained (see Table 4), their final adjustment being made by intensity calculations.The determined structure of these crystals was discussed comparing with those of Sr (HCO2) 2and Ca (HCO2) 2, of which the latter corresponds to D152h-Pcabwith the unit cell containing eight chemical units. In all these crystals (HCO2) - ion possesses approximately the same structure, its two C-O distances being practically the same, thus showing almost complete resonance between the two valence bond structures for (HCO2) - .App. The thermal expansion coefficients of anhydrous strontium formate along the three principal axes were measured by the method of the high angle X-ray reflection. The values between 23°-76°C are : aa= (3.57±0.08) ×10-5, ab= (1.15 ±0.05) ×10-3, ae= (2.36±0.05) ×10-5.