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8. (15 points) The NaCl and NiAs binary structures can both be described as close-packed structures of anions (ccp and hcp, respectively) with cations filling all the octahedral holes. However, compounds with strong ionic character almost always adopt the NaCl structure, while the Nias structure is only observed for a few compounds with more pronounced covalent character. a. Consider the geometry of adjacent octahedral holes (containing cations) in the NaCl and Nias structures, shown schematically below. In the NaCl structure, adjacent cations share two nearest-neighbor anions, so the distance d between them is the diagonal of the square of side length a (the ionic bond length). This can be determined by the formula: d = 2a? In the NiAs structure, adjacent cations share three nearest-neighbor anions in an octahedral face. To determine the distance between them, we can use the formula for the height of a pyramid where all 3 angles at the vertex are 90°: h? = a/3 Then, the total distance between two "pyramids" will be twice their height: d = 2h Suppose an ionic compound has a bond length a of 300 pm. Calculate the distance d between adjacent cations in the NaCl vs. the NiAs structure. b. Use your results from part a to suggest a reason why compounds with strong ionic character prefer the NaCl structure over NiAs. C. Compare the binary compounds calcium selenide and cobalt(II) selenide. Which of the two would more likely be able to adopt the NiAs crystal structure? Explain your reasoning. NiAs (nickel arsenide) structure NaCl (rock salt) structure