We provide methods and apparatus for preparing crystalline-clad and crystalline core optical fibers with minimal or no breakage by minimizing the influence of thermal stress during a liquid phase epitaxy (LPE) process as well as the fiber with precisely controlled number of modes propagated in the crystalline cladding and crystalline core fiber via precisely controlling the diameter of crystalline fiber core with under-saturated LPE flux. The resulting crystalline cladding and crystalline core optical fibers are also reported.

A thermoelectric material containing a dichalcogenide compound represented by Formula 1 and having low thermoelectric conductivity and high Seebeck coefficient:

R.sub.aT.sub.bX.sub.2-nY.sub.n (1)

wherein R is a rare earth element, T includes at least one element selected from the group consisting of Group 1 elements, Group 2 elements, and a transition metal, X includes at least one element selected from the group consisting of S, Se, and Te, Y is different from X and includes at least one element selected from the group consisting of S, Se, Te, P, As, Sb, Bi, C, Si, Ge, Sn, B, Al, Ga and In, a is greater than 0 and less than or equal to 1, b is greater than or equal to 0 and less than 1, and n is greater than or equal to 0 and less than 2.

A thermoelectric material containing a dichalcogenide compound represented by Formula 1 and having low thermoelectric conductivity and high Seebeck coefficient:
R.sub.aT.sub.bX.sub.2-nY.sub.n(1) wherein R is a rare earth element, T includes at least one element selected from the group consisting of Group 1 elements, Group 2 elements, and a transition metal, X includes at least one element selected from the group consisting of S, Se, and Te, Y is different from X and includes at least one element selected from the group consisting of S, Se, Te, P, As, Sb, Bi, C, Si, Ge, Sn, B, Al, Ga and In, a is greater than 0 and less than or equal to 1, b is greater than or equal to 0 and less than 1, and n is greater than or equal to 0 and less than 2.