Heater for retrograde solvothermal crystal growth, method of making, and method of use

An apparatus for solvothermal crystal growth, the apparatus comprising:

• a pressure vessel having a cylindrical shaped portion having a vertical orientation and extending between a top end and a bottom end, wherein the pressure vessel is configured to process a material in a fluid at a pressure above about 5 megapascal and less than about 2000 megapascal and temperatures above about 200 degrees Celsius and less than about 1500 degrees Celsius;
• a cylindrical heater having an upper zone and a lower zone that are each disposed around the cylindrical shaped portion;
• a growth chamber having a bottom end surface, a cylindrical shape, and positioned within the cylindrical shaped portion of the pressure vessel;
• a bottom insulating member below the bottom end surface;
• and
• at least one bottom end heater, positioned between the bottom end surface and the bottom insulating member, wherein the bottom end heater includes a center, an outer radius that extends from the center, an inner region with an inner radius that extends from the center, an outer region between the inner radius and an outer radius, a ratio of the inner radius to the outer radius is at least about 3%, and is configured to provide a varying power density within the bottom end heater whereby a power density within the inner region that is at least 25% less than a power density in the outer region, wherein
• the varying power density within the bottom end heater is provided by at least one of a heating element having a larger cross section, in a direction perpendicular to electrical current flow, within the inner region than within the outer region, a heating element wherein a spacing between adjacent segments is larger within the inner region than within the outer region, or at least two independently-controllable hot zones.