De Broglie Wave Lenght And Number Of Waves K

I could not understand very well because of the argument between me and the teacher about k space and wave lenght of de broglie because he said that we can not explain solid state or lattice or crystal only by suing k space or vector k not by using de broglie wave lenght because de broglie wave lenght is not easy to measure or not that real but i donot know can anyone tell me why we can not use de broglie wave lenght when we talk about soild state and instead of that we use ((k wave number)) or wave lenght=2*3.14/k
to explain solid state and crystal???????

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