The two cubic oxides ReO3 and BaRuO3 both adopt a cubic perovskite type structure; however whilst ReO3 displays Negative Thermal Expansion (NTE) between 10 and 250 K BaRuO3 displays Positive Thermal Expansion (PTE) over the same temperature range. The occurrence of NTE in ReO3 has been explained using the concept of Rigid Unit Modes. 

(a) Explain, using appropriate diagrams, why most materials display positive thermal expansion. (4 marks)

(b) What are the key features of Rigid Unit Modes that allow for NTE? (5 marks) 

(c) Given both ReO3 and BaRuO3 contain the same structural frameworks what is the likely cause of the absence of NTE in BaRuO3? (4 marks)