  Ground-state and finite-temperature properties of a special class of exactly
solvable Ising-Heisenberg planar models are examined using the generalized
decoration-iteration and star-triangle mapping transformations. The
investigated spin systems exhibit an interesting quantum behaviour manifested
in a remarkable geometric spin frustration, which appears notwithstanding the
purely ferromagnetic interactions of the considered model systems. This kind of
spin frustration originates from an easy-plane anisotropy in the XXZ Heisenberg
interaction between nearest-neighbouring spins that favours ferromagnetic
ordering of their transverse components, whereas their longitudinal components
are aligned antiferomagnetically.
