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We present the structural and magnetic properties of KNaCuP$_2$O$_7$investigated via x-ray diffraction, magnetization, specific heat, and $^{31}$PNMR and $^{23}$Na NMR measurements and complementary electronic structurecalculations. The temperature dependent magnetic susceptibility and $^{31}$PNMR shift could be modeled very well by the uniform spin-$1/2$ Heisenbergantiferromagnetic chain model with nearest-neighbour interaction $J/k_{\rmB}\simeq 58.7$ K. The corresponding mapping using first principles electronicstructure calculations leads to $J^{\rm DFT}/k_{\rm B} \simeq 59$ K withnegligibly 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 anone-dimensional uniform spin-$1/2$ Heisenberg antiferromagnet. Thetemperature-dependent unit cell volume could be described well using the Debyeapproximation with a Debye temperature of $\Theta_{\rm D} \simeq 294$ K,consistent with the heat capacity data. The diverging trend of the NMRspin-lattice relaxation rates ($^{31}1/T_1$ and $^{23}1/T_1$) imply the onsetof a magnetic long-range-ordering at very low temperatures supporting theanticipated $T_{\rm N} \simeq 0.38$ K from the inter-chain couplings. Moreover,the NMR spin-lattice relaxation rates show the dominant contributions fromuniform ($q=0$) and staggered ($q = \pm \pi/a$) spin fluctuations in the highand low temperature regimes, respectively mimicking one-dimensionality of thespin-lattice. We have also demonstrated that $^{31}1/T_1$ in high temperaturesvaries linearly with $1/\sqrt{H}$ reflecting the effect of spin diffusion onthe dynamic susceptibility. Further, the inter-chain frustration alsosubstantially impede the magnetic ordering rendering the spin-lattice a perfectone-dimensional uniform spin-$1/2$ Heisenberg antiferromagnet over a widetemperature range.

Quasi-one-dimensional uniform spin-$\frac{1}{2}$ Heisenberg antiferromagnet KNaCuP$_2$O$_7$ probed by $^{31}$P and $^{23}$Na NMR