Calculate the lattice energy of ammonium carbonate
The lattice energy is not directly measurable, and it is difficult to calculate. In a paper about estimating lattice energies, the estimation for $\ce{Na2CO3}$ is 30% off from the reference value (which is from CRC Handbook of Chemistry and Physics). I mention sodium carbonate because ammonium carbonate is not one of their examples. The introduction explains why it is not possible to directly measure the lattice energy:


  Lattice  potential  energy  (UPOT)  is  a  dominant  term  in  the thermodynamic analysis of the existence and stability of ionic solids.  Direct  experimental  determination  is  generally  not possible since, in practice, the crystalline solid dissociates into atoms  and not into  gaseous  ions,  as  is  required  in  the  lattice energy  evaluation.  Therefore,  its  indirect  experimental  determination, computation, or estimation is of considerable interest in modern materials science; indeed, whenever the energetics of condensed-state materials are studied, the chemical processes under consideration may be rationalized if the appropriate lattice energy steps can be incorporated into the thermochemical cycle.


They go one to discuss how you can obtain the lattice energy from experimental or theoretical data:


  A  variety  of  estimation  methods  for  lattice  energies  is vavailable. These include the Born-Haber-Fajans thermochemical cycle (which requires ancillary thermodynamic data) and modern computational methods (which generally require knowledge of the lattice constants and the coordinates of the ions as well as needing an established force field). The computational methods  range  from  direct  energy  calculational  procedures through to programs that produce lattice energies in the course of their modeling of the solid. Quantum mechanical procedures are also available, but are highly computationally expensive and are generally applied only to the simpler systems.


Then they talk about cheaper methods that are less accurate. The paper is about a new quick estimation methods.


  By contrast,  a  few  exceedingly  rapid  estimation  methods based on ionic radii or volume per formula unit (“molecular”volume) have  been  developed.  Originally  only  applicable  to the simpler binary ionic solids (such as MX (with a 1:1 charge ratio)), the volume-based approach has recently been extended by  us to  include  MX2(2:1)  and  M2X  (1:2))  salts.


So check the CRC Handbook of Chemistry and Physics.