Do completely ionic solids exist

The possibility of assessing the ionicity of atoms in crystals from the diffraction pattern has been among one of the most controversial topics of X-ray analysis4 

11 Electron-density difference maps on Li2BeF4 calculated with all reflections sin 6/1 = 0.9 A 1 (81 K). (a) Based on the neutral atom procrystal model, (b) based on the ionic model. Contour levels are drawn at intervals of 0.045 eA 3.1 Full lines for positive density, dashed lines for negative and zero density. The standard deviation, estimated from [2Lff2(F0)]1/2N, is 0.015 eA-3. Source Seiler and Dunitz (1986). 

5 In the original publication, the contour interval was erroneously specified as 0.015 eA 3, P. Seiler and J. D. Dunitz, private communication. 

James 1948, in his famous treatise on X-ray diffraction, stated that 

Seiler and Dunitz point out that the main reason for the widespread acceptance of the simple ionic model in chemistry and solid-state physics is its ease of application and its remarkable success in calculating cohesive energies of many types of crystals (see chapter 9). They conclude that the fact that it is easier to calculate many properties of solids with integral charges than with atomic charge distributions makes the ionic model more convenient, but it does not necessarily make it correct. 

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