Thermal expansion in BaRuO perovskites - an unusual case of bond strengthening at high temperatures.

The temperature dependences of the structures of three polytypes of BaRuO have been investigated between room temperature and 1000 °C using high resolution synchrotron X-ray diffraction. The structural studies reveal a systematic decrease of the Ru-Ru distance as the pressure required to prepare the polytype increases. The O-O distance across the shared face increases as the Ru-Ru separation decreases. The 9R and 4H polytypes undergo unexceptional changes with increasing temperature. In 6H-BaRuO there is an apparent increase in the Ru-Ru interaction at around 650 °C and a concurrent reduction in the O-O distance, indicating an anomalous strengthening of the Ru-Ru interactions upon heating.