Powder samples with the general composition (Tl,Hg)₂(Ba,Sr)₂(Sr,Ca)Cu₂O_8-d, abbreviated 2212, were synthesised by solid-state reaction of metal oxides in closed silica ampoules. Depending on the relative amounts of Ba, Sr, and Ca, the superconducting transition temperature, Tc, varied significantly (45-100 K). Increasing the Ba content raised T_c, which was measured by susceptibility and resistivity methods. From Rietveld refinements based on X-ray powder diffraction data, minor atomic rearrangements were detected, which could be correlated with the superconducting properties. Tc also increased with decreasing Cu valence, as calculated from the bond valence sum.

Single crystals of the two compositions, Tl_0.8Hg_0.7Sr_1.9Ca_1.1Cu_2.5O_8-d and Tl_1.2Hg_0.8Ba_0.9Sr_1.5Ca_0.6Cu₂O_8-d, were grown by melting the 2212 oxide mixture under high Ar pressure. A rare distortion within the superconducting Cu-O layer was discerned from refinements based on single-crystal data, possibly indicating movements of the CuO₅ polyhedra.

Selected-area electron diffraction experiments on some of the prepared 2212 phases mostly revealed ordered structures, but occasional streaking was found along c*. Using high-resolution electron microscopy, a stacking fault of 2201 structure was observed in the 2212 matrix.

Na- and K-doping of 2212 was tried, but no alkali ions were found by energy-dispersive spectrometry in the formed crystallites. A Tc change in the Na-containing sample could be attributed to oxygen overdoping caused by the Na₂O₂ addition. After reduction, Tc(onset) was near 112 K.

A sol-gel synthesis route, based on metal acetates and tartaric acid in water solution, was used to prepare the precursor "Ba₂Ca₂Cu₃O_x" aiming at superconducting (Hg_1-xTl_x)Ba₂Ca₂Cu₃O_8+d (1223); the synthesis yielded a homogeneous powder. The decomposition of some precursors and two types of superconducting 1223 oxides was studied by thermogravimetry experiments. After oxygen anneal the Tl-containing compound Hg_0.9Tl_0.1Ba₂Ca₂Cu₃O_8+d exhibited a record-high Tc(onset) of 140 K, as measured by resistivity methods.