Sapphire materials diamond anvil cells

Diamond is the hardest material known to man. The hardness of diamond on the Mohs hardness scale is 10. The second hardest material is corundum (sapphire [AI2O3]), which has a hardness of 9. Corundum, as well as diamond, is transparent to visible radiation. The real difference in hardness is quite large because the absolute hardness of diamond is 7,000 Knoop and of the corundum is 2,000 Knoop [24]. Bulk modulus of diamond is 4420 kbar [25] but is only 2400 kbar for synthetic sapphire. However, because the cost of sapphire is much lower than that of the diamond and also sapphire is more transparent to UV light, sapphire anvil cells (SAC) are often used for an optical cell for pressure <100 kbar. Sometimes cubic zirconia anvils are also used for pressure <30 kbar [26]. 

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