Polycrystalline chemical vapour deposition synthetic diamond material

A polycrystalline CVD synthetic diamond material comprises an average thermal conductivity at room temperature through its thickness of between 1700 and 2400 W per m per K, a thickness of at least 2.5 mm and a visible transmittance through its thickness of at least 25%. The material may form a central area of a polycrystalline synthetic diamond wafer, which may be substantially crack free over said central area. A method of forming a polycrystalline synthetic diamond wafer by microwave CVD comprises controlling an average temperature of a refractory metal substrate to between 750 and 950 degrees C, maintaining a temperature difference between an edge and a centre point of the substrate to no more than 80 degrees C and, during a growing stage, increasing the pressure of a process gas and reducing an average power per unit area of the refractory metal substrate. The diamond material may be used for heat conducting applications, such as in disk lasers.