Exploration and Optimization of Tellurium‐Based Thermoelectrics

obtain good quality crystals. Take for example the Zintl phases Ba2Sn3Sb6 [118] and Ba3Sn4As6, [119] which
were reported as not accessible from simple stoichiometric powders, however with excess Sn, typically
on the order of equal atomic parts Sn versus the compound, [120] excellent crystals of both were obtained
following dissolution of the tin matrix in dilute HCl. Homogeneity is greatly improved due to the
presence of a high‐temperature liquid during the reaction process; solid state reactions on the contrary,
require direct face‐on‐face contact for a reaction to occur, increasing reaction time and making
uniformity difficult – sometimes impossible – to obtain.
It should be noted that metals possessing low melting points such as Sn, Pb, Ga, In, etc. can be
very effective flux materials as described above in the 2005 review by Kanatzidis et al. [120] Somewhat
more care is often necessary with non‐reactive metal fluxes as the common practice of washing away
with dilute acid can also affect the target compound assuming it has similar stabilities. Due to the
nature of the major compounds studied, these fluxes were generally avoided for a number of reasons:
The structures are already doped with similar elements, precise stoichiometries are much more difficult
to achieve/prove when containing unknown metal concentrations and there is a wide selection of stable
binary tellurides formed by means of metal‐flux reactions (i.e. PbTe when using a Pb flux).
Alternatively, binary salts can be used as a flux in a reaction. Though individual binary salts such
as KCl have relatively high melting points, eutectic viii combinations of two salts allow chemists to
precisely choose a melting point that best suits the reaction. Tables such as Table 3.1 are therefore
routinely used in a solid state chemistry laboratory to optimize crystal growth. Advantageously, most
binary salt matrices can be easily dissolved in water to obtain the reaction crystals and with most
materials herein, these are inert fluxes or non‐reactive. There are a few disadvantages regarding the
use of salt fluxes that include efforts afterwards to remove the salt matrix such as washing the ingots
with water or applicable solvents; this of course adds the potential risk of the sample reacting with the
solvent in the process. If crystal growth is the only intention, it is optional to remove the salt, but one
must always be aware of potential alkali‐doping within the target crystals and would need EDX (6.2) or
another analysis technique to prove the flux was truly inert in each case. The salt’s elements should
always be considered when attempting to solve the forthcoming crystal structure should they react –
special care with Li‐based salts is advisable since Li is nearly if not impossible to see via standard EDX or
XRD techniques.
viii A mixture of elements or compounds that possess a lower melting point than the individual components.
34