  We present the structural and magnetic properties of KNaCuP$_2$O$_7$
investigated via x-ray diffraction, magnetization, specific heat, and $^{31}$P
NMR and $^{23}$Na NMR measurements and complementary electronic structure
calculations. The temperature dependent magnetic susceptibility and $^{31}$P
NMR shift could be modeled very well by the uniform spin-$1/2$ Heisenberg
antiferromagnetic chain model with nearest-neighbour interaction $J/k_{\rm
B}\simeq 58.7$ K. The corresponding mapping using first principles electronic
structure calculations leads to $J^{\rm DFT}/k_{\rm B} \simeq 59$ K with
negligibly small inter-chain couplings ($J^{\prime}/k_{\rm B}$, $J^{\prime
\prime}/k_{\rm B} < 0.1$ K), further confirming that the system is indeed an
one-dimensional uniform spin-$1/2$ Heisenberg antiferromagnet. The
temperature-dependent unit cell volume could be described well using the Debye
approximation with a Debye temperature of $\Theta_{\rm D} \simeq 294$ K,
consistent with the heat capacity data. The diverging trend of the NMR
spin-lattice relaxation rates ($^{31}1/T_1$ and $^{23}1/T_1$) imply the onset
of a magnetic long-range-ordering at very low temperatures supporting the
anticipated $T_{\rm N} \simeq 0.38$ K from the inter-chain couplings. Moreover,
the NMR spin-lattice relaxation rates show the dominant contributions from
uniform ($q=0$) and staggered ($q = \pm \pi/a$) spin fluctuations in the high
and low temperature regimes, respectively mimicking one-dimensionality of the
spin-lattice. We have also demonstrated that $^{31}1/T_1$ in high temperatures
varies linearly with $1/\sqrt{H}$ reflecting the effect of spin diffusion on
the dynamic susceptibility. Further, the inter-chain frustration also
substantially impede the magnetic ordering rendering the spin-lattice a perfect
one-dimensional uniform spin-$1/2$ Heisenberg antiferromagnet over a wide
temperature range.

Materials Science